Depending on what you are cutting, uniform rough sawn lumber is pretty much a pipe dream. You may get uniform thickness in plantation grown, properly thinned hardwoods, but naturally grown trees are NEVER uniform, and the wood quite often moves while being sawn. Ever watched a 12" flitch bow while you cut boards off it? I have, many times.

Not only that, only perfectly quarter sawn lumber every shrinks uniformly while drying, and even then will sometimes go wonky.

Wavy bandsawn lumber is a result of improperly sharpened or tensioned bands or over-feeding (just like a small bandsaw re-sawing, any excess pressure results in the blade being forced off true), but even then those little portable bandsaws are nowhere near rigid enough to keep that small band from following the grain. Just the nature of the beast. Large stationary bandsaws can wander, too, I've found.

Naturally, a decent sawyer should be on the lookout for this kind of stuff, and saw appropriately. If you have a flitch that won't saw clean one way, flip it. Keep the log set so that the grain is as straight as possible down the log, etc. Helps a lot to know what you are doing, and keeping in mind the nature of wood, a skilled sawyer will get you much better lumber than someone who's sloppy or careless!

My brother had a FoleyBelsaw circle mill for more than 20 years, sawed quite a bit of lumber on it. Learned a lot, too, the hard way.