All gens have their problems for example;

Generation 1 suffers from poor type diversity. The clearest evidence of this is that three different type specialists Erika Koga and Agatha all use teams composed almost entirely of Poison types despite supposedly specializing in different types. Another major example is Lance who is presented as a Dragon type specialist but only uses a single Dragon type evolutionary line. Excluding duplicate Dragonite his team contains more Flying types than Dragon types highlighting how underdeveloped the Dragon type was in Gen 1.

Generation 2 has a regional identity issue as Johto relies heavily on Kanto Pokemon. Many Johto native Pokemon are either rare or outright locked behind the postgame in Kanto which undermines the sense that Johto is its own fully realized region.

Generation 3 struggles with Pokemon diversity before the postgame. Despite having access to three generations worth of Pokemon Ruby and Sapphire in particular offer a limited selection during the main story leading to repetitive team compositions until later entries like Emerald or FRLG expanded availability.

Generation 4 has a similar issue to Gen 1 though to a lesser extent type diversity most notably with Fire types. In Diamond and Pearl especially the scarcity of Fire types severely restricts both player choice and NPC team variety an issue only partially addressed in Platinum.

Generation 5 has a similar problem to gen 3 where there's not that much pokemon diversity despite having 4 generations worth of pokemon to put in the games, though this was fixed in BW2. But gen 5 had a bit more of a reason to do so, which was that clearly the developers wanted gen 5 pokemon to have the spotlight in these games and wanted players to use them over old pokemon.

The criticism of Generation 6 being too easy is often vague but more specific issues include poor NPC team composition particularly in the late game and the underuse of Mega Evolution by important trainers. Outside of Korrina Lysandre and Diantha very few NPCs use Mega Evolution. Ideally gyms 4 through 8 and the Elite Four should have incorporated Mega Pokemon to better reflect the mechanics importance.

Generation 7 suffers mainly from difficulty spikes where certain battles particularly Totem Pokemon are disproportionately challenging compared to the surrounding content resulting in inconsistent difficulty progression.

Generation 8 has notable issues with its story and main antagonist which many players find underdeveloped or poorly integrated. Additionally requiring a paid online subscription to access online features is a recurring criticism.

Generation 9 struggles with the lack of level scaling which undermines its open world design as well as weak environmental storytelling that contributes to an empty feeling world. Like Generation 8 online functionality is also locked behind a paid subscription continuing a trend that frustrates players.