CSD has nothing to do with UX. CSD just means the application draws the window controls and borders.
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.
Don't all WMs have shortcut keys anyway?
I gave up when even normal settings were hidden and I had to use Gnome Tweak Tool, but when I upgraded, I found all my changes had been wiped.
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.
Combined headerbars? UX decision.