Network transparency. X11 has it. Wayland doesn't. Wayland's devs tend to handwave the problem, either claiming it will somehow be implemented once they work on the other laundry list of things they want first, or claiming it's a niche requirement nobody wants or uses.
Well, it is a niche requirement and they simply do not care about the few users - even denying that they exist (I use it everyday and remotely over ssh and it works well for me). The aim of all these efforts in not the desktop anyway, but mobile or embedded devices. For the desktop Wayland will have no advantage. But they still somehow convinced a lot of people who do not understand anything about how that X somehow limits performance of the graphics stack so it must be replaced.
On top of that they're doing the #1 thing you're not supposed to do in development: completely rewriting a working system.
If they would just rewrite something it would be ok with me. The problem is breaking compatibility at the protocol level. This is really stupid.
X11's main flaw is that it's supposed to be inefficient. It might be, but I've never noticed any significant difference between user interface performance on Ubuntu vs Windows or Mac. I think much of it is "This sub-nanosecond operation that is only called once or twice every frame takes THREE TIMES AS LONG under X11 as it should!" type purism.
There is no fundamental benefit with respect to performance as Wayland and modern X clients basically work in the same way when operating locally. Somehow people believe that the old rendering APIs supported by X for backwards compatibility somehow prevent modern clients to do things efficiently. This is completely untrue as X has been extended with modern interfaces. There could be some performance benefit because Wayland basically merges the X Server, window manager, and the compositing manager. Of course, this could be done in X as well without breaking the protocols.