Just don’t do it.
If you have dozens of separate communities scattered across a plethora of platforms, it’s tempting to bring them together onto a single, better, platform.
You could manage them better, give them better service and it would certainly be more efficient.
The problem is that these communities quite like their own unique, quirky, identities. They don’t like being treated like several other communities. They’re quite familiar with the platforms they’re on and don’t feel a need to change.
So don’t consolidate, it’s just too risky.