Maybe a more concrete example will answer your question. The Wayland protocols are found here [1]. Your compositor will implement some of these protocols and not others. If I’m writing an application I will; possibly through some toolkit, ask the compositor what features it supports and configure how my application works accordingly.
This isn’t particularly different to say, an toolkit not supporting a particular form of input, such as GLEW not supporting tablet devices or certain GPU’s having more extension methods — which you can see if you look at Khronos specification. Or an even more drastic example, is an application having both a terminal display and a X11 display like Emacs.
So, it's quite possible that one gets a distro or a machine set up so everything works locally, but you can not tunnel a GUI application or run a remote desktop there. Or that you get a machine where you simply can not take a screenshot to report a bug?
So... I was one of those people rooting for anybody to replace X11 when wayland launched. But given the current situation, I'm just hopping that Red Hat finishes dying off before I have to work with somebody stupid enough to set some GUI system that I need to access, but only runs on a computer where I can not run a GUI.
Well yes — in the same way that you could have had a half-baked implementation of X on a remote desktop; but it’s unlikely because most GUI-based interfaces will bundle with some level of support. For example, I don’t expect Canonical’s Wayland compositor to not support screenshots; so while I don’t think your worry is unfounded, I do think it’s disproportionate.
Oh, ok. I imagine you had a stricter interpretation of "universally adopted" then.
If every practical system support them, they are universal enough.