I like the Gen 5 starters in terms of design (Samurott is awesome imo, Serperior has a nice, slick, and beautiful design, Emboar I'm pretty indifferent towards though), but the issue I do have with them is their battle capabilities. I find that in battle, particular in-game, they are pretty underwhelming. I'm not sure what ultimately happened with them, but stat wise they seem to try to go for a jack-of-all-trades build but end up becoming a master of none in the process. And in doing so, their in game performances are overall very mediocre.
Samurott is the best of the three for an in-game run, and is an overall good Pokemon imo and I don't think it's inherently bad, per se, but at the same time it's also not too interesting. It's considered the worst of the Water-type starters in battle, and that's not because it underperforms, but because it's practically white bread amongst the brethren of Water-types. Compared to the likes of Swampert and Empoleon before it, who had epic second typings and the stats to make them great tanks, Samurott just comes off as a bit underwhelming in that regard. Even moreso when compared to the three who came after, Greninja, Primarina, and even Inteleon, as Gren and Primarina have great second typings, the former is insanely fast, and Primarina has a lot of raw power to back itself up plus good special bulk and Calm Mind. Inteleon even stands out a bit more in battle because it's just fast and strong and goes for a clear direction, while Samurott is just a mixed attacker but isn't fast enough to really sweep. Its typing is also pretty bland because it's pretty similar to Feraligatr who was in the game just prior to Samurott's, HGSS, so that immediately just makes Samurott less interesting considering it has many similarities to Feraligatr in that regard. It's the least egregious of the Unova Starters in the grand scheme of things, because in spite of being generic it's still a good Pokemon in-game, just not spectacular.
But then you get to, say, Serperior and this mon just underwhelms in-game. Lack of immediate power combined with a shallow movepool means it can't really sweep like other starters can, despite being fast, and it relies on a very slow gimmick to sweep with Coil+Leech Seed+Leaf Blade. This takes a while to get going, and while this is effective late game it's not exactly fun, which is what a starter should be first and foremost in-game. Its matchups in-game are also really bad, all the way starting from Burgh and except with Clay, it struggles against just about every Gym Leader. At least other Grass-type starters like Chesnaught have coverage to have a fighting chance in their games. Serperior? Nah. It's pretty lacking in both offense and support, if it had something like Poison Powder or Sleep Powder like Meganium and Venusaur did in the past, or the offensive prowess of Torterra, then maybe it would've been good in-game, but nope. It's just a mediocre setup sweeper that relies too much on its setups and doesn't have any coverage to back itself up. When you have to rely on Return as any semblance of coverage that's pretty sad.
Emboar is also mediocre, and in some ways I've found myself to have even worse experiences with it in-game than Serperior. It has a strong early game up to Burgh, but from there it's just bad news for the Tepig line all the way. Its stuck with its best Fighting STAB being Arm Thrust for a long time, and its best Fire STAB is Flame Charge, plus Pignite is very slow and not all that bulky. Elesa, Clay, and Skyla all wipe the floor with it. It evolves into Emboar, and except for Brycen, things still don't get better for it. Its best STAB is either Hammer Arm from the relearner, which is nothing compared to Infernape's Close Combat from the gen prior, or Brick Break. It lacks a good Fire STAB outside of Heat Crash which is very inconsistent, and perhaps Flamethrower which comes from Emboar's weaker offensive stat, which doesn't come until Level 43. This is quite bad. Emboar also has the awkward combination of being both frail and slow, which is never a good combination to have on a Pokemon, especially a starter. It struggles against half the Elite Four, and N and Ghetsis don't pull any punches either. Its best move doesn't come until Level 62, which is post game, and Emboar's best moves, Flare Blitz, Wild Charge, and Head Smash, all demand that Emboar hurt itself, if not KO itself in the process, which is bad.
The other issue with them, especially with Serperior and Emboar, is that they are ironically outclassed by other normal Pokemon of their types even in the context of their own game, especially BW1. Unova is not short on Grass-types, and almost all of them are better than Serperior in-game except Amoonguss and Whimsicott. Right after you get two badges you can get Petilil and Sewaddle, and both Lilligant and Leavanny are better than Serperior because they have more immediate power, while Lilligant has Sleep Powder to incapacitate an opponent and set up while needing fewer turns and hitting harder in the grand scheme of things. Later on Sawsbuck and Ferrothorn become available, and both are also much stronger late game than Serperior as Sawsbuck has STAB on Return, more power, and good coverage in Jump Kick, Megahorn, and Wild Charge and later on Swords Dance as well. Ferrothorn is slower, but it also has many more resistances, more bulk, more immediate power, and options such as Gyro Ball, Payback, and Bulldoze to cover what its Grass STAB cannot. Meanwhile with Emboar, Darmanitan straight up murders any reason you would want to use Emboar. Darmanitan is stronger, faster, gets Flare Blitz and Hammer Arm/Superpower earlier than Emboar and naturally, and has Sheer Force to boost its power even more while not having some of the glaring weaknesses Emboar has. Emboar is also a mediocre Fighting type and on that half, you have more specialized Fighting-types like Conkeldurr, Sawk, and Scrafty who have better Fighting STAB and more tools at their disposal to make them competent threats, moreso than Emboar. Samurott is the least outclassed of the three, but that's also partially because Unova is pretty dry on available Water-types as they are either too rare (Simpour), have some issues (Seismitoad), come too late and underleveled to be of efficient (Jellicent, Basculin), or just plain suck (Alomomola).
When you're outclassed by other normal Pokemon of your type, that's pretty terrible. Contrast that to the Kanto starters: Venusaur and Charizard are by far the best options of their types since Victreebel and Vileplume are both watered down Venusaur, while Arcanine and Ninetales rely on being stone evos and that comes with its issues. Blastoise has more competition, but it has the edge of coming early and not needing a shit ton of grinding to be useful. When it comes down to it, no matter which starter you choose, you're getting the best end of the deal for that type. Not every starter has to be like that, but many of them are at least very fun and unique with neat stat spreads and movepools that make them fun, reliable, and dependable so they feel good to use.
And a starter need not have secondary typings either. The Sinnoh, Kalos, and Alola starters benefit for sure, and combined with their specialized roles makes it so that in those cases, all three starters are fun and splashable to use because they have nice movepools and typings (moreso Sinnoh and Kalos, Alola starters suffer from being slow as sin), and Hoenn's starters also benefit as well. But look at the Galar starters for instance: all three of them are single typed, but their stats are slightly min-maxed to optimize them for a particular role, and they have fun signature moves and are fast/strong enough to be reliable for an in-game playthrough. The Johto starters are among the weaker starters, but in the Johto games themselves they are competent enough to do their assigned roles with grace.
In my eyes, at least, a Starter Pokemon should first and foremost be a fun and splashable Pokemon to use in game. A starter is supposed to be your partner and friend whom you rely on as your most trusted ally in your journey, and many of them nail it by being good, solid, and reliable Pokemon, and in many cases quite fun to use. But the Unova starters completely fail in this front: they have nice designs, sure, but that doesn't make up for their mediocre battle capabilities as all three of them are lacking in some front: one is pretty generic and boring, one is both weak and lacks a good movepool, and one has the bad combination of being frail and slow while all of its best moves demand that it hurt itself. It sucks even more when they are arguably outclassed by better Pokemon you can find in the grass, especially in BW2 when there's even less reason to keep them because of the increase of available options.
Now I will concede: I don't think Serperior, Emboar, and Samurott are inherently bad Pokemon. But as starters, they are pretty poor. There's a distinct feeling that I should get from using a starter Pokemon in-game and with the exception of Samurott, the Unova starters just couldn't get that feeling down right. Samurott is the only one that's a good Pokemon in-game but in the grand scheme of things it's arguably the least interesting Water starter thus far in terms of battle capabilities.
