If anything, the game's Trainer battles almost became... too monotonous lately? We've seen so much of the patterns and the lack of actual strategy from too many opponents to the point it do comes off as easier for veterans. You can only start to see actual strategy from rare NPC trainers or late Gym Leaders, Elite 4 (if any) and Champion. And even then, not late game opponents are winners, if Wulfric is any indication.
This is something I hear often, it having "become" this way or heavily changed, and I don't aree. Gym leaders have been filled with gunk Pokemon and movesets since Gen 1. Even late-game. I figure Gen1 and Gen2 don't need that much elaboration (Sabrina Abra lol), but...
R/S Leader(s) 7 has two Pokemon
total, and for a double battle no less.
R/S Leader 8... Seaking is neither the worst nor the second worst out of 5. Luvdisc and Sealeo get those honors.
D/P Leader 7 has Snover and Sneasel.
B/W Leader 7 has Vanillish. No leader in the whole game has more than three Pokemon.
All of these Pokemon are worse than any 6-8 XY leader's Pokemon. Sure, Wulfric gets obliterated by any Fire-type, but so does B/W Brycen, unless you fall before the might of... Beartic's Brine.
GF only needs to make complete and competent (but not 100% optimal) movesets
This may have not been what you meant, but a lot of people point to the "better strategies" and "complete movesets" of Pokemon from
earlier generations, and, well...
Winona (6) has
Water Gun / Supersonic / Aerial Ace (50 Atk!) / Protect Pelipper. Send help, please. Skarmory has Sand-Attack and Fury Attack.
All of Wallace's (8) Water-types have Water Pulse and no stronger STAB move (besides Earthquake Whiscash, which is admittedly good stuff). Luvdisc has Flail, Milotic has Twister (covers Dragons? it has Ice Beam. Twister just free reprieve chance), and Seaking has Fury Attack. All free-turn filler.
Candice's (7) Sneasel has Avalanche (???) and Slash. Snover has Razor Leaf and Ingrain (sure it'll survive to take advantage of that). Abomasnow has Swagger and Grasswhistle?? Like sure, those two can be dangerous, but this is clearly cheese and not a competent moveset for rewarding play.
Volkner's (8) Ambipom has no STAB moves. Raichu and Luxray have Charge Beam but no other special moves. Octillery has no move above 65 BP, which includes 10 BP Bullet Seed.
Skyla (6) has all-physical Swoobat with 57 Attack. Not technically all-physical because... Amnesia? (not Simple.) Unfezant has Quick Attack, Razor Wind, and
Leer.
Brycen (7) has Astonish Vanillish, Rapid Spin Cryogonal, and Swagger Beartic.
B/W Drayden/Iris (8) have two Dragon Dance sweepers! With, uh, Dragon Tail as their only STAB move. What? And Fraxure has Dragon Rage.
"Why are you only using the first entries and not Emerald/Plat/B2W2?" Because X was a first entry too. There may be a case that remake trainers are notably better than first entry trainers, but that's not the narrative I normally hear.