IVs and breeding overall are a huge, unnecessary bummer. What a barrier for entry to the average fan who wants to play competitively.
The most annoying this is just the grind to play online period, honestly. That is, that stuff above Level 50 will be automatically reduced to Level 50, but if you're below that? Your level remains whatever it is. So you have to get to at least Level 50 to get online. Which doesn't sound that bad at first, 'cause what, it only takes a few minutes to get a Pokémon up that high if you grind the Elite Four or whatever, right? But when you want to try out a bunch of different stuff and see what works and what doesn't, having to get each one to Level 50 just to be able to use them at all gets tedious really, really, quickly. And that's without touching on stuff like EV training or breeding (for egg moves), which I don't think should actually go anywhere due to the diversity of builds and options they add for some many Pokémon that just isn't worth sacrificing at all just to make getting a competitive Pokémon a bit more speedy. Having those options and diversity matter so much more to me, especially when there's no way around grinding regardless.
Basically, no matter what they do, stuff like Pokémon Showdown will be better for trying stuff out and seeing what you like and don't like. Then once you find it, actually take the time to make it in the actual games and use it there. But testing stuff out on the main games themselves will never really be worth it for me, in comparison. Just too slow and tedious to make new team members and switch stuff out and get them ready for online, no matter what they do.