1. Shitty debugging if something close to the event passing layer doesn't work like it should
2. It's hell for multithreading. Writing multithreaded Java with a swing GUI? I hope you like writing wrappers around every single fucking function to check if you're in the EDT or not.
3. It's not actually event driven, it's really just polling.
3a. Switching between reactive and imperative structure* is irritating, especially when you know that it's all actually imperative, so effectively you're switching programming styles just to satisfy the whim of whoever wrote the library.
*I've only ever seen event-driven programming implemented as a sub-paradigm of an imperative language