Centralization. Meta will develop the fediverse protocol up with their own specific enhancements, locking most of us onto their version of the platform due to convenience given enough time. (Think RCS as an example, it's technically an open standard but Google has effective control of it thanks to special features)
You might say "Well, if they are a better platform, so what, why not go there", but the problem is that they can cut off the rest of the fediverse as "outdated" essentially privatizing everybody.
Honestly, this kind of thing should be done automatically by whatever app they sign up with. It's not something that the average person will really know how to choose.
Because it's not only number of users, it's also number of connections to other servers, how many federate with it. Most people don't wanna use a social network where they have to type exactly what they want in to search it every time. Never mind the different policies about posting and de-federation that can completely change how the network works for each user.
It's the disadvantage of decentralizion, and we've gotta band-aid it somehow.