I saw 5 out of 7 LaRue scope mounts loosen on .308 guns in a Shrek "Heavy Carbine" class that I hosted in late 2013. John claimed that he saw similar results in every "Heavy Carbine" rifle class he teaches regardless of how tight the LaRue mounts are set-up due to the extra pressures inside of the .308s. My Bobro mount cracked at the cross bar that rests between the rail sections after running it thru the Shrek class on a .308 LaRue PredatAR.
For 5.56 guns I have never seen one loosen that was set-up properly. They are fine on 5.56 ARs. For .308 ARs I would only recommend a fixed/non-QD scope or optic mount.