It's not just "some" caveats: then auto-unwrapping means that you cannot use then as fmap reliably. In JS there is no such thing as Promise<Promise<Number>>. You cannot await/then the outer promise so that you can do something with the inner promise.

In practice it's not too much of a problem, but it does mean that if you use some FP library, you cannot trivially wrap a promise to make it a monadic type that will work with the library's monad utilities.