If you're using legendaries like Phione and Regigigas in OU, of course you're going to get owned. If you're using horrible sets that include moves like Hyper Beam and Giga Impact and have no EVs, of course you're going to lose. The most useful legends (Mewtwo, Kyogre, etc.) are banned from OU in the first place because they are too strong, but legends like Thundurus, Heatran, and Latios can be integrated into a team including non-legends and will work very well in the hands of a competent battler.
Yeah, I'm sure a competent battler can make good use of them. My point was, non-legendary pokemon are equally useful in the hands of these same competent battlers, if not more useful. The best way to beat other competent battlers is to throw things at them they most likely have not battled dozens of times already. Half the battle is being able to predict what your opponent is about to use, and keeping one step ahead of them.
https://discord.gg/BDZcvd9t72