>Gen 8
>Main gimmick mechanic is Dynamax
>doubles your HP
>changes all your attacking moves to Max moves or G-Max moves
said Max/Gmax moves have a variety of powerful effects with a variety of BP depending on the original attack, and perfect accuracy
>buffs apply to your allies and debuffs apply to your target's partner
>Gmax moves have completely new effects for most users
>changes your status moves to Max Guard
>makes you immune to phasing
>makes you immune to being locked into a move
>even with Choice items, but just disables them
>completely immune to weight based moves
>immune to destiny bond
>immune to flinching
>Pain Split, Super Fang, and berries work as if the user has its normal HP, effectively making them half as effective
>Removes Substitute
>lasts for an arbitrary 3 turns
>is completely convoluted, often complained about by several people for constraining strategies in battles

>Gen 9
>Main gimmick is Terastallization
>You just change to a mono type based on what your Pokemon's Tera Type is
>If your Tera Type matches your primary or secondary typing, you get a stronger STAB
>You can also use Tera Blast after Terastallizing, this game's version of hidden power (which was gone and changed to be completely different)

GameFreak definitely realized they went overboard with Dynamax and made a much simpler gimmick. It's also likely to add way more depth than Dynamax too.