There are various criticisms of tokio, coming from different directions. Some have to do with the fact that some abstractions in the futures ecosystem are leaky today and that makes them less easy to use than they could be (though they won't always be leaky[]). But others have to do with understanding the internal implementation of these abstractions - people who feel they must understand how their library works internally before using it.

Of course, schedulers are just complicated. Most of the time you don't think about how complicated your scheduler is, because its either an OS primitive in the kernel or a language primitive in your language's runtime. But since tokio is a library - and modular - it gets criticism for being complex that in my opinion is unfair.

[] To be more concrete: a future is essentially a state machine representing the stack state at any yield point; it can't (currently) contain lightweight references into itself because they'd be invalidated when the future is move around. This means using borrowing in futures programs is often infeasible today. Solutions are in the works.