I personally dislike the Gen 6 gyms and RS/ORAS gyms a lot. I've said elsewhere that the latter is just them using a very strange and restrictive selection that extends to Wattson not even having Manectric, but the Gen 6 gyms are arguably worse for it. As I recall the vast majority had a single new pokemon alongside two Gen 1 Pokémon, and even beyond that selection; even the eighth gym leader only had three Pokémon. I'm not a big fan of small teams in general even around Gym 5, but Wulfric should have had a team of 5 or even 4. As-is I think the Gen 6 leaders are easily the most forgettable and half-hearted leaders in the series.
So the 3 pokemon holdover from gen 5 is absolutely bad and the fact they just did
nothing with the gym leaders stands in stark contrast to gens 4, 5 ,7 AND 8
but in terms of the actual generation composition I think gen 6 isn't terrible.
Viola: Gen 3, Gen 6
Grant: Gen 6, Gen 6
Korrina: Gen 5, Gen 1, Gen 6 (& her other battle had Lucarios but eh)
Ramos: Gen 2, Gen 1, Gen 6
Clemont: Gen 5, Gen 1, Gen 6
Valerie: Gen 3, Gen 1, Gen 6
Olympia: Gen 5, Gen 2, Gen 6
Wulfric: Gen 4, Gen 5, Gen 6
That string of gen 1's is a bit brow raising but the picks themselves aren't hideous and it's just the middle 4 gyms. It honestly feels like a coincidence which is fairly rare for me to say wrt gen 1 stuff. & I think they specifically leaned into a variet of generatiosn (Grant is the only exceptoin here) because of how dang huge the kaloss dex is & hogw small gen 6 was.
Honestly I kind of like the picks over all, there's some interesting stuff in there like Viola's Surskit, Ramos' Jumpluff, or Olympia's Slowking. Even some of the gen 1 picks are kinda neat. Mr. Mime was used by Sabrina but it's always an eclectic pick and seeing it instead of Gardevoir as the psychic/fairy representative is kind of fun.
*sigh* maybe in a Z they would have had
4 or, dare I say it,
5 Pokemon by the end and we'd have even more variety though...speaking of
Gen 4:
Roark: Gen 1, Gen 1, Gen 4
Gardenia: Gen 4, Gen 4, Gen 4
Maylene: Gen 3, Gen 1, Gen 4
Wake: Gen 1, Gen 2, Gen 4
Fantina: Gen 4, Gen 1, Gen 4 -> Gen 3, Gen 1, Gen 4
Byron: Gen 4, Gen 2, Gen 4 -> Gen 1, Gen 2, Gen 4
Candice: Gen 4, Gen 2, Gen 3, Gen 4 -> Gen 2, Gen 2, Gen 4, Gen 4
Volkner: Gen 1, Gen 4, Gen 2, Gen 4 -> Gen 1, Gen 2, Gen 4, Gen 4
The gen 1 picks dont stand out as much when the last 2 leaders have 4 pokemon (Roark & Gardenia having 3 also helps) and when they actually do pull on other gen 4 Pokemon, but conceptually a split of Pokemon generations isn't that outlandish. I think it's fun that Gardenia has a full set of gen 4 Pokemon, a leader having nothing but their generation is fairly rare (gen 1 & BW1 excluded, of course), especially once you leave the 2
And Gen 8 because I was thinking about it:
Milo: Gen 8, Gen 8
Nessa: Gen 1, Gen 8, Gen 8
Kabu: Gen 1, Gen 1, Gen 8
Bea: Gen 2, Gen 5, Gen 8, Gen 1
Alister: Gen 5, Gen 7, Gen 8, Gen 1
Opal: Gen 1, Gen 3, Gen 4, Gen 8
Gordie: Gen 6, Gen 2, Gen 8, Gen 8
Melony: Gen 8, Gen 5, Gen 8, Gen 1
Piers: Gen 5, Gen 6, Gen 4, Gen 8
Raihan: Gen 5, Gen 3, Gen 8, Gen 8
Touch and go for a bit there but having 4 pokemon for 5 gyms (& also 2 separate leaders for 4 & 6) helped distribution a lot, and I think having 8 gens under their belt helps it point out more. Gen 8 also made a point to have a number of families that line up with the types, which helps a lot too. Though I think the lack of dark & Dragons for gen 1 for Piers & Raihan might've helped but still!
Incidentally I liked the gen 6 leaders in the anime. They got to show off more personality and strategies and different Pokemon. the XY anime really pulled a lot f weight in general but that's for little things I like thread