Any amount of unevenness or out of level will affect the flow over it. But by what amount is what's in question. Your spillway could be seriously out of whack, and with enough water running over it still look fine. It's when you turn it down that this becomes an issue.
If you know you're only going to be running your waterfall at full speed, then it can be off by quite a bit and still look fine. I just know, if I had one, I'd want at least a few settings:
- Off, to heat the spa,
- high for maximum waterfall affect,
- medium for normal circulation, and
- low for a little background ambiance while snoozing at the pool, or while sitting around chatting with someone when I don't need Niagra Falls roaring a few feet away.
I'd want my spillway to look perfect in all modes, and not be forced to keep it on high just to mask some poor workmanship. I'm sure every builder can site a different tolerance for what is considered acceptable. But, IMO, since it can be just about perfect, it should be just about perfect, as it is just a matter of skill and effort applied during installation.
Since I don't have one, I can't give you what I would consider an acceptable tolerance range. Maybe someone else here, with a spillway, can weigh in on this subject...