There is one thing in Gen 7 I really don't like. It just took me a while to realize how much I dislike it. This is something that existed in S/M but it wasn't until I played Ultra Moon that I realized how bad it was. Chances are it has been talked about before in this thread. Or rather, I am quite sure it was talked about back when S/M were new. But I wanted to say something about it as well.
I'm talking about the SOS mechanics. I don't like them at all. I'm just so annoyed by them that I find it hard to tolerate their existence. I'd say they are one of my four biggest annoyances in Gen 7, the others being the lack of training spots (S/M), the Rotom Dex's endless babbling (US/UM) and the lack of a National Dex within the games.
What makes the SOS mechanics so annoying? Many things. To start with, they are highly based on luck and unreliable. For instance, the wild Pokemon can call for an ally when you don't want it to. This is annoying if you are trying to catch a wild Pokemon as this means you need to defeat one of them first since you can't throw balls when there are two opponents on the field. I remember once in S/M when I was trying to catch a Magby. Before I could catch it, I had to defeat around 15 ally Magby that it called up before it finally failed to call one which allowed me to catch it. It is also annoying when you want the wild Pokemon to call for an ally... and it doesn't. Or it does, but it fails. Or, if you are looking for a Pokemon that is exclusive to SOS calls and it takes forever for it to appear. Getting the SOS exclusive Pokemon is something I find extremely annoying and I am really not looking forward to having to do it one more time. SOS chaining also requires a high level of concentration on what you are currently doing so that you don't accidentally do something stupid you aren't supposed to. I don't like having to think all too much while just looking for a specific wild Pokemon so this annoys me a bit.
I don't know what it is like to try to obtain shinies, HAs or Pokemon with multiple flawless IVs through SOS chains and I surely don't want to know. Thankfully I don't need to do any of this either since I'm not interested in shinies, HAs can be traded for or sent over from previous generations (depending on the Pokemon), and flawless IVs can be obtained through breeding. Previous generations had things like Friend Safari, Hordes, DexNav, Hidden Grottoes and Dream World which gave you access to one or multiple of these things, and I definitely prefer these things over the SOS mechanics.
In Gen 6 and 7, I found it annoying how they removed several good and useful features from the previous generation, or changed some for the worse. But I would definitely not mind if Gen 8 removes the SOS mechanics permanently and replaces them with something better.
I know that they changed the SOS mechanics a bit in US/UM so that any regular Pokemon can only ever call for one ally unless you use an Adrenaline Orb, but one is still one too many for me. It has annoyed me too many times throughout the game. It should have been so that they can't call for SOS allies at all without an Adrenaline Orb. Maybe that would have been a bit less realistic from a game world perspective, but a lot better from a gameplay perspective.
Is there anything positive to say about the SOS mechanics? Yes. I don't mind the Totem Pokemon calling for allies, they are meant to be challenging and giving them allies in battle helps with that. Also, SOS chaining Chansey at Poni Plains is a good training spot, I'll give them that. In fact, combining that with the Rotom Exp. Power and the existence of the Gym has made it quite a bit easier to train in US/UM compared to S/M, which I really appreciate. They are still quite far below the epic levels of Gen 5/6 though.
I need to rant about one other thing as well. I think the annoyances with the Rotom Dex in US/UM has been talked about enough in this thread, but I thought about one thing that I wanted to say about it too. When Rotom is talking, it's mouth is constantly moving and the text is also scrolling at the lower screen. I find this annoying as it seems to draw my attention from the upper 3DS screen. When something is constantly moving and I am focusing on something else, it seems like my eyes tries to get focused on what is moving and I lose focus on whatever I am currently trying to focus on. Don't know if anyone feels the same way, but I think that's how it is for me at least.
I guess I have a similar annoyance with the following Pokemon in HG/SS and how they are always constantly moving instead of standing still even when the trainer is standing still. Guess that's another of the many things I dislike about the following Pokemon in HG/SS, it steals my attention from whatever else I am trying to focus on. In comparison, I didn't have a problem with Pikachu in Yellow or following Pokemon in Amity Square in D/P/P because they at least stood still when the trainer did.
I read some recent posts here about the annoyances with traveling through Ultra Space and entering the wormholes to find legendaries there. After trying it myself for a bit as I am currently trying to catch the Ultra Beasts, I completely agree with these posts. I don't have much more to add to what others have already said. Just some small things. I dislike how the corridor you are riding through has transparent walls which makes it hard to see where it ends and where it continues. I guess you get better at this mini-game the more you play it but it still feels very luck-based since the placement of the obstacles, speed boosters and wormholes (as well as their level to an extent) are decided randomly every time you enter. I like catching legendaries in the games but this whole thing makes me not look forward to doing it in US/UM quite as much as I would have liked to.
With all of this said... I like US/UM and I like S/M too but it feels like they suffer from quite a few poor design and gameplay choices that drag down the overall experience a bit. Which is a shame since these games could have been so much better if it weren't for these things.
On another topic that was discussed recently, I like the Moor of Icirrus and the Challenger's Cave in Gen 5. They might not have any real purpose but at least they have wild Pokemon, items and even trainers, which is something I never say no to. That alone makes them fun to explore for me. I think Pokemon needs more optional areas like these and others, not every place in the games needs to have some sort of specific purpose in order to exist.