As soon as you properly fix down a profile rail it wont move, which is clearly an advantage, however if the rail is bent to start with you'll still have a bent rail.

I just checked the straightness of two profile rails and a SBR20 rail on my surface plate to check. Took readings 3 times for each over 400mm length in different places so should be reliable...

SBR20-400mm:0.03mm height variation.
Hiwin 15mm (760mm long): 0.06mm variation.
NSK 20mm (520mm long): 0.05mm variation.

The hiwin catalogue says +-0.1mm for normal precision height, but I don't think it states weather that is +-0.1mm for random rails, or +-0.1 over a single rail.

This agrees with my saying the the straightness is similar, but absolutely no reason to think SBR rails are better as they're clearly not!