In my opinion it's just easy to do wrong. Most uses I've seen (at least in java land) are attempts at non-blocking io which ends up turning the whole application into observables from bottom up. Which in turn makes your app hard to debug and reason about.

When done right, when you actually need the observer pattern, and use it as an event queue I'm sure it's probably amazing though.

Those are good points but I agree with the other commenter that these are just difficulties with asynchronous programming generally. I like that Rx makes reasoning about those issues more straight forward and something you must handle instead of something that will bite you later if you didn't think through it.

this is the problem with async programming in general not just rx. Relevant read:

https://journal.stuffwithstuff.com/2015/02/01/what-color-is-...

Yes I know about colored functions. I'm looking forward to loom and virtual threads on the jvm to solve that.

BUT it doesn't change the fact that Rx is hard to implement correctly and is often misused when you don't need the observer pattern.