Box of Bunnies
Banned
It's not a detriment because you can simply not use it. We don't even have all the details on how it works yet lol
Before, you could use EXP Share to EV train a Pokemon without having to send it in and switch out to something else every single battle. EXP Share changing to EXP All could obstruct that. It's not "don't use it", it's "something you used to use may not be viable any more."