I think among other issues would be the Gmail-ification and iMessage-ification of the fediverse. What I mean by that is open standards like email are dominated today by many people using Gmail accounts as it is popular, “free”, and comes with a ton of features. Then google started “walling off their garden” by adding features that only work between gmail accounts. Similarly, apple also took the open standard SMS and started adding on features only available between other iPhones.
What we might see is some of the coolest features the fediverse has ever seen, but it will come at the cost of most users ignoring or dealing less with "irrelevant" things not on meta ran instances.
Hope we can resist such a change, but that is what I am concerned about.