Well, more like the Cinnamon DE isn't. And, yeah, if you stray from the two superpower DEs with the most dev polish (KDE and GNOME), you're asking for jank and bugs.
https://bugs.launchpad.net/ubuntu/+source/unity/+bug/1308572
If you go back to an older GNOME version you can probably also find the current Cinnamon bug in there, it is in libcaribou after all.
Cinnamon at least makes operating like a classic DE a first order priority and it works nice across multiple screens.
KDE does the same but aesthetically KDE drives me crazy for some reason starting with the complete lack of standardisation on padding and going down hill from there.
- There's no hibernate button. You can hibernate... by running "sudo pm-hinernate" on terminal.
- There's no session save, like, really.
- pm-hibernate sometimes fails without a single error message. Turns out it would refuse to run when the kernel was updated, OK fine that makes sense, except that the error message is redirected to /var/log/something and hidden beneath hundreds of lines of noise.
The only nice thing about it is that fractional scaling mostly works, which is nice, because I happen to have one 1080 and one 4K monitor. I'm sorely tempted to move to KDE but then I couldn't figure out how to make fractional scaling work there.
It's a total mess.
Note how starting from Plasma 5, KDE doesn't officially come with a screen saver installed anymore, and only has a screen locking mechanism (kscreenlocker, which isn't exempt from its own share of bugs BTW).