I think what frustrates me the most with X/Y is that the section from the start up until Gym 3 is so good but then, in my opinion it just falls off a cliff completely after Korrina. Like, Reflection Cave is honestly one of the toughest dungeons in all of Pokemon, especially during a Nuzlocke. That trainer with a Throh and a Hawlucha (pet peeve here but why is a random trainer using the next Gym Leader’s ace?!) is no joke as well as the one with Absol and Pinsir and all the wild Shadow Tag Wobbuffet. However, after that I just find the game gets so much easier. I’d probably rank Valerie, Olivia and Wulfric as three of the worst Gym Leaders in the whole series to the point where I can’t really remember what their teams are off the top of my head. Wulfric especially is a complete joke.
The XY gym leaders are wild. WHY ONLY 3 MONS? Many of the other decisions in those gyms are good, but there's no reason for a gym past 3 to be running fewer than 4 mons, and the E4 should have full teams.
Viola I would argue is poorly designed. Inverting type matchups in gym battles is good, but I specifically think that should be avoided in gym 1. Gym 1 should involve NOTHING clever. She is a reasonable challenge, which is shocking and laudable from both a 1st gym and a bug gym, but this is someone's first gym, act like it.
Roark is pretty tough. Rock/Dragon to prevent the auto-lose to water, Rock/Ice covers some things that resist his STAB, solid stat spreads, but you also have access to plenty of options to beat him...Great gym 2.
Korrina is an embarrassment to the title of Gym Leader and I have no idea who approved any decisions here.
Ramos exists, I guess? You can only do so much with a grass leader, but he at least leans into what the type does well.
Clemont is great design IMO. You encounter him directly after a bunch of ground types become available, and there's plenty of training available for your Gible or Trapinch. And a ground will do well against him, but isn't an auto-win due to Emolga. The only real problem here is that he's Gym Leader 5 and only has 3 mons, which is unacceptable, but I'd still say he's the best-designed Electric specialist in the series. Not from a power perspective, but from an "accomplishes the goal of making sure the player understands the game" angle.
Valerie is actually fine. Mawile ignores Fairy's weaknesses, Mr. Mime sets up screens, and Sylveon sweeps. Good demonstration of the different Fairy-types available, good compensation for their weaknesses. Now if only she had more than 3 mons!
Olympia has reasonably bulky Psychics with support moves and coverage. It's actually close to what I'd do for a Psychic leader in the early-midgame, before you expect a team of 5ish mons.
Wulfric is arguably the worst final leader in series history. Well, that's unfair. He leads with Snow Warning, then has the most specially-bulky and physically-bulky Ice types in the game. IDK if you can improve that. But fundamentally he's an ice-type trainer in the lategame with only 3 mons, there is no way to make that a losable fight.
There are usually actual teams there. There's some element of strategy or intentional design. But they seem to have applied the breaks hard in some areas and they actually come out as incredibly weak teams.