Earlier quoted context omitted.
This isn't that different than X though which also has the fire off a message and receive event back. Very few of the functions actually make you want (even in xlib, which wraps the protocol itself to be a bit easier to use from C, the majority of functions still are async - much to the chagrin of newbies trying to decipher error messages)
What do you mean by all Wayland's calls being blocking though?
It isn't really Wayland's calls itself, I prolly stretched too much there, but the main thing is the async discussion usually comes up in the context of running applications remotely, which Wayland simply doesn't support. If you do a loop of XInternAtom (which btw you shouldn't do, even in xlib - notice that there's also XInternAtoms, plural, for that kind of thing which batches them to reduce wait time...) locally, it is unlikely to matter. The time spent there is near zero anyway. But remotely, now you might be looking at several ms per iteration and that adds up fast. Now the blocking aspect can become problematic.
Since Wayland doesn't support this kind of network remote use anyway, the distinction doesn't matter. Local X vs Wayland are both fast.