A more Wayland-oriented remote desktop protocol would probably make for an even better VNC alternative, but I don't really know why SPICE never got the uptake it deserved.
This approach is fundamentally limited in terms of performance/responsiveness, as you're ultimately just trying to "guess" what's happening and apply best-effort techniques to speed things up, falling back to just using a video stream.
A proper remote desktop solution like RDP on Windows works in cooperation with the OS's GUI subsystem in a way that makes the RDP server aware of the GUI events, so it doesn't have to guess, and can offload compositing and some 2D operations directly to the client, rather than sending rendered bitmaps.
Thus it didn't catch on because it focuses on a narrow use-case nobody should be using except in emergency/break-glass situations (you should instead be remoting into the VM directly, for reasons explained above), and even for such situations, it didn't offer anything substantial over VNC, except everyone and their dog has a VNC client by now, but good luck finding a functional SPICE client for anything but Linux.
I personally had issues with the project years before that when I tried to install their Linux .deb and they ran `pip install` as root in the pre install script inside the .deb. That caused so much havoc to clean up I was pissed at them for years. Now that idiocy is blocked by default in current versions of pip.
I wonder if anyone has forked it...
https://www.cvedetails.com/cve/CVE-2024-25140/
Personally, I'm not exposing my installations to the Internet, so I feel relatively secure regardless.