The most important feature of the secure desktop is one that is rarely ever mentioned: you can hit CTRL+ALT+DELETE on a real UAC prompt. This is something similar Linux prompts lack (as Linux lacks a similar privileged shortcut sequence); GNOME has full-screen, top-level, UAC-like prompts for PolKit and sudo, but it's hard to figure out if it's real or a fullscreen application.

There are ways to avoid the CTRL+ALT+DELETE safeguard, but it's a lot harder than it is to spoof a password prompt that looks like the OS one. It's kind of unfortunate that KDE went with the macOS/Windows XP approach rather than doing what Windows does.