Well, OT in general does not need the server but server-less implementations are more complex (more transforming functions to write, except of "inclusion transformation" you also have to write "exclusion transformation" algorithms).
I also wouldn't say that "OT is very simple to implement" - it is in it's base form, for linear data, with the server in the network. But every enhancement brings a lot of complexity.
If two clients, your client (a) and another client (b), writes the letters a and b respectively at the same time, from either client's perspective they where first, so on client A the state will be "ab" and on client B the state will be "ba". How do you solve that without a server/master ? With a server/master the server just have to increment a counter for each operation, and the clients can use that counter to know which order. So if a has counter 77, and b has counter 76, the state will be "ba".
Of course, it depends on the collaborative editing solution. When it comes to OT -- if the implementation is correct, it doesn't really matter how many operations are queued for synchronising. So, it doesn't really matter how long users were de-synced when creating their content. But this is only theory and it applies more to technical side as there might be some semantical problems (intention preservation).
I believe the same is true for CRDT but I am not sure.
So, on one hand - if the editor is working fine for small batch of changes, it should also work fine for big batch of changes. The reality is often more harsh, though, and full of edge case scenarios.