Personally I dont really play online so maybe that's why I don't like them so much. Tera is my favorite gimmick though because it gives you so many options and on every single pokemon. In theory, a defensive tera at the right moment can make any pokemon viable really because it happens at the start of the turn. It also let's you give pokemon that are crying out for certain types what they deserve, like Luxray and dark type (he still kinda sucks, though). It also gives you an extra slot to hold your tera type so you're not wasting an item, and you can do it with any of the six which allows you to adapt. Plus, you can do stuff like tera poison with something you don't mind sacrificing and swap in and out to soak up toxic spikes and stuff like that.
I don't know, to me, the other gimmicks just purely buff your pokemon (not you, mega Garchomp...) But tera only really helps if your strategy pays off and that's kinda cool to me. I quite liked the tera raids as a change of pace aswell sometimes to break up the gameplay loop, although like I said I don't do online so I just pulled in bots.
Then again, I'm the guy who said they not only liked Let's Go but wanted a sequel, and I'm a gen wunner, so maybe I'm just retarded.
Anyone else like tera the most?