Lightdm can have arbitrarily complex greeters too, by the way.
Or the system admin could do the responsible thing and test the theme _before_ setting as default.
It is the responsibility of the theme maker to test - picture a bigger deployment with varius systems, surely you don't expect the admin to get one of each just to test a login manager. Near none of the themers will do so, and shaders written on Nvidia cards will likely crash and burn on any compliant platform.
This is a real risk, and I guarantee you will see that happen if you go there.
Anyway, we shouldn't restrict features just because someone can mess up. I want to have a beatiful login manager at my computer and doing my part to realize this. If this is not suitable for some other use cases, then they shouldn't use it. One size doesn't fit all.
For your use case system admin could select a simpler theme without shaders, or go for another display manager.
Implemented maui login manager mockup as an SDDM theme: