Not all styli have erasers, but a very substantial fraction do. Detecting it is a basic operation when you’re using the Pointer Events API (event.button == 5 and event.buttons &== 32). I use not supporting this as a proxy for other basic errors of implementation, such as using mouse events instead of pointer events. This is why I say it’s a very bad sign. It’s a simple thing that I can point at and say, “this thing is half-baked; they have very probably done a bad job”, without needing to go through a larger battery of comparative and subjective tests.

The eraser not working is a sign the app hasn't been thought out from the perspective of graphic tablet users, which would likely be a pretty major target demographic.

Sure, but not to the level of a "very bad sign". Especially when the whole thing is sluggish to begin with and seems to be struggling with far more basic operational tasks.

If it's sluggish to begin with and fails basic operational tasks, sure, it doesn't matter that much whether the eraser works—because it hasn't even met the low bar to be judged on its tablet functionality at all.