> why not? what's the difference between those two categories, mentioned in your last two sentences, as far as this argument about illegal states is concerned? not clear to me.

I kinda began my comment with that reason: The difficulty entirely depends on whether discarding the occasional invalid write is possible. If you can simply return an error and ignore the write/transaction, you're golden.

If you can't, it becomes incredibly complicated

Shouldn't an unrepresentable bad state not even have been proposed as a write tho? I mean the way I understand it, if something is trying to write a bad state somewhere, it is being represented somehow isn't it?