Heres my 2 cents on this as a SCAR 17 owner. From actual personal experience, i tossed a TA11 on it in a Larue mount. Ive had the same setup on other rifles (m1a, ar, etc.) and never bothered to rocksett the optic to the mount or tighten the hell out of it. On other rifles it was never an issue but the SCAR had a way to bang it loose. Now after going through proper per manual install ive shot hundreds of rounds without an issue on the SCAR. I absolutely DO think the SCAR is harder on optics, but as others of said with an ACOG or quality optic (mounted properly), its unlikely you will have major issues.
For another note, the machine shop that does my SBR engraving also makes custom NVG setups. They have had multiple failures on the SCAR 17 and in an attempt to solve the issue they purchased a bunch of from what I understand to be "blemish" SCARs for testing purposes. Their stock NVGs hit the skids almost every time on the 17 after 2 or 3 thousand rounds. The prevailing theory was that it had something to do with the PWS brake, but what they ended up finding was that the bolt returning to battery produced some type of vibration that was hard on things sitting on the rail. When they went back to the drawing board with their NVG's they were able to reinforce certain areas including the tube housing itself which eliminated the issue. All in all, everyone loves the rifle but you have to be exceptionally choosy with optics, especially if you are in the Night vision game. On a weirder note, it seemed even worse when the rifle was suppressed once they learned what to measure for. Go figure!
After having the rifle for a few years, I wouldnt tell anyone not to buy one, its a damn cadillac of a .308 rifle. As long as you stick to optics proven track you will have the finest combat rifle out there in my opinion.