If, as an application developer, you target X11, you have a program that will work on Linux and BSD with all desktop environments. It'd even work on Wayland via Xwayland. If you want to use it on other OSes, it's less smooth, but also working on MacOS via Xquartz, and Windows via Xming. There's even an X11 compatibility layer for Haiku (Xlibe).

With Wayland, you don't even get compatibility with Gnome and KDE. You need conditional compilation to get mouse events if you port to FreeBSD Wayland.

For the medium term, if your goal is to reduce fragmentation, X11 is the portable target, even if you use Wayland.