I don't have a strong opinion on whether they're on-topic or not, but they often go like this:
Question: Is X balanced?
Answer: No, here's what would need to change for X to be balanced.
(Here's where things go horribly wrong...)
Question edited: Is X balanced now that I changed it?
That's not something that works in the SE format. It undercuts answers, undermines the voting system, and so on.
I'm posting this because it happens so often. Understandably, homebrew creators don't want to stop at "is version 1 balanced", they want to go through the process and have a balanced end product. And that highlights a problem with this type of question - it really works better as a back-and-forth dialogue, rather than a static question-answer. That makes them another thing forums do better than us, which doesn't necessarily mean that they're off-topic, but it is a potential reason why they maybe should be.