This is a little obtuse, but something you're going to want to be aware of and monitor. And you have to catch this at the right time during the build, or it'll literally be "in stone."
If a water fall, or overflow or whatever, is not built really well, and perfectly level, a little PB trick to hide that fact is to turn up the pump RPMs to create more flow over it. This can conceal an underlying build defect, like an uneven or not-level edge. The increased flow pushes more water over the edge, a thicker layer of water, that disguises the defect. It all looks great, you sign off on it, and the PB drives away. It is only later that you realize you have to run your pump on high all the time or else your water fall or perimeter overflow looks crooked because the water only runs over it on one side.
You prevent this from happening by discussing it with the PB ahead of time, and making your expectations well known: that the edge must be perfectly level and that you want it to look even with a minimum amount of flow. Just letting him know you know about this might be all it takes. Or you might have to go out there with a level and check it yourself before everything is finalized (this would have to happen before you fill the pool). It's not reasonable to expect it to be perfect to 1/32" inch, it's plaster and tile after all, but it should be level enough that it works well with a relatively low pump speed. (Didn't know what you were getting into, did you?) And, unfortunately, this needs to be checked before there is water to check it! Like right after they install the finish surface (tile or stone). Hopefully the sub that will do this work knows his stuff, but that is not a guarantee.