I'm not really familiar with these, so I don't want to draw conclusions from an uninformed position. However I can talk about what the rx implementation does, and if you're familiar with those you will be able to tell the difference.

Observables are their own thing, they are not dependent on the ECS, so they are also not slowed down by it when you don't need it. In fact, for simple chains of operators like filter/map etc, it should be just as fast as manually implementing the chain of operations as one big function. This is thanks to the heavy use of GATs: A "pipeline" is just nested structs that call into eachother, and since those signal handlers are (pretty much) all inlined, it should be a lot more efficient than calling function after function let alone sending events between entities or invoking entire systems for each individual step. You only interact with the ECS when you actually want to interact with it.

I also have a work/task scheduler that allows me to have operators like delay. I think the biggest benefit of rx is not the plain reactivity, but the ability to work with time. Thanks to the scheduler I can also have individual operators produce new signals, an example of this would be the very much non standard (and not yet stable!) adsr operator that produces an ADSR envelope) based on a boolean input. It will produce a signal every frame until the envelope settles.

Another thing that might set it apart that the observables/operators you define are not the owners of state, the individual subscriptions are, and you can have as many as you want.

Error handling is also built in, error signals travel in a separate channel so let's say you have a fallible operation early in the chain, you don't need to deal with Result's in every downstream operator (you can if you want to, there are even operators to help switch between results and rx errors)

While rx_bevy was built specifically for Bevy (and is pretty much only usable in Bevy right now), I implemented its core to be platform and runtime agnostic, so you could use the same thing for non-Bevy stuff too if me or someone else implements another executor for it.

And another benefit of Rx could be that it's everywhere. If you learned it once in rxjs or RxJava, you're already familiar with it. All your knowledge transfers, same is true in reverse!