Maybe not unstable, but can easily cause usability issues e.g. too large/small font, similiar foreground/backgound colors, undesired effect on UI widgets, etc.

Plus, they are contained in a deb package which run scripts as root on the device. For example, you wouldn't want me to be able to upload a theme to extras without testing that has "rm -rf" in the postinst script (not that I would, obviously!)