On Wayland, the compositor can either let windows draw their own borders, or it can disallow it. On X11, if a window want's to draw its own decorations... it will just do it, and you won't be able to do anything about it.
Pretty much every single Wayland compositor follows this except for GNOME, who refuse to do so. It causes problems with windows like Davinci Resolve who don't draw their own decos. This leaves some windows without any controls at all lol. But this is a tangent and purely a GNOME problem.
I mean yeah... But most people are going to have trouble with that, especially being unable to resize or move the window. I even saw a tech Youtuber try out gnome and they were baffled on why they couldn't move or resize davinci resolve whatsoever. Its almost shameful how negligent and hard-headed gnome is being.
Sure it does. Allowing windows to draw their own decorations, often in different ways and with different styles and themes, and not respecting the settings in xfwm4 as to what window-control buttons should be drawn (and where)... that's a huge UX issue.
The process drawing it's own decorations? Technical decision that both Windows (tad complicated) and macOS already do, nothing to do with UX. There's nothing stopping this model from having the uniformity of SSD (as like on macOS), but it does on Linux due to multiple toolkits which don't all agree with eachother.