good =/= broken. I know it's popular on this site to say "Low-risk high-reward" and "difficult to punish" but those statements don't really mean anything and can apply to a lot of the top-OU pokemon.
I think a lot of pro-ban people- myself included at times- haven't properly articulated why Genesect, a case of "good" is worthy of being banned when Tapu Lele, Ash Greninja, and Megagross are running riot in the tier.
Let's look at this way.
What do those three aforementioned Pokemon do?
Tapu Lele does shitloads of special damage and sometimes surprises the blobs with a Psyshock, right? It's also a pretty big threat to Mega Sableye. It's also reasonably fast.
Ash Greninja also does shitloads of special damage. Water/Dark is a pretty good STAB combination, if I may say.
We all know what Megagross does- tons of physical damage (Courtesy of Tough Claws and 145 base attack) and then some. Also fast, and annoyingly bulky in part due to a great typing.
Now that we've gotten that out of the way, how's this:
Do those aforementioned Pokemon do anything else beyond that? Tapu Lele can run either specs or scarf sets or a surprise Expert Belt set. Alright, next. Ash Greninja can run Specs for maximum deeps, or it can run a Z-move/Expert Belt for the surprise factor, or a Life Orb for consistent (But risky) extra damage. Next. Megagross runs Meteor Mash/Iron Head, and three moves of its choosing. It does work with those other three moves, but you know it's going to be Ice/Thunderpunch in there, probably Zen Headbutt to take advantage of Tough Claws and STAB, and Earthquake, and maybe Bullet Punch is thrown in for some bizarre reason.
Long story short, you KNOW what they do. There is a limited amount of variation. Like some anti-ban people have noted, this may reduce the metagame to chess, which is a fucking bullshit comparison, because to compare something to chess implies equal starting conditions known to both competitors, something that I'm pretty sure is mostly impossible in Pokemon because there's only 6 slots in a team and much, much more than 6 possible members to fill those slots.
Let's turn this back to Genesect:
Can you tell me what Genesect does?
1. Amazing Choice user, even Band sets have Extreme Speed for priority to make up for the lack of speed and Blaze Kick for surprise not-Flamethrower fire coverage. (You can even run Ice Beam for the lulz and spank those filthy Landoruses/Gliscors/Garchomps that want to switch in on you)
2. Runs great setup sets, especially with the right Download boost. That movie Genesect is broken as hell when it's running Shift Gear and the physical walls not named Ferrothorn and Skarmory are out of the picture.
3. Runs great mixed attacking sets, with Expert Belt to bluff a Choice item and Life Orb for the "fuck you" factor.
4. Has something to beat ALL its theoretical counters in OU and therefore can run a lure move on the mixed sets, or any set really if you want. Nice Heatran, now eat my Hidden Power Ground. Nice bulky Ground/Water you got there, now take my Giga Drain and die. Cool Mantine bro, now take this Thunderbolt and gtfo. Toxapex? Sure, specially defensive ones will probably take multiple Thunderbolts (without a Download boost), but... does Toxapex have any offensive presence?
Oh, you're running something that you pulled from other tiers to counter it, congratulations, you likely screwed yourself over in the process. Nice Amoonguss/Chandelure, mate, you're... running Amoonguss and Chandelure. Granted, they ARE solid counters to Genesect's usual antics, it's just that you're running Amoonguss and Chandelure.
Tl;dr: There are probably worse Pokemon in OU right now that
do what they do better than Genesect. But on the other hand,
they are generally limited to that specific role. Genesect is limited to no single role, and each version of Genesect is viable and has different checks.
I really don't want to spam this line, but I don't see anybody refuting it:
It's true that you can figure out Genesect's set relatively quickly. That does NOT mean you're carrying its check.