While it's true that a lot of legendaries are disappointing, and occasionally even nonsensical (i.e. the Gamma Gun of Ghoul Slaying), some are pretty handy.
Two Shot is easily the best modifier. It's extremely powerful with any alpha-strike weapon like a Gauss Rifle.
Instigating (+100% against full health enemies) can be pretty good, though maybe not if Suvival level enemies are very bullet spongy.
Assassin's (+50% against humans) and Mutant Slayer's (+50% against Super Mutants) are good. Any +50% is situationally good, but those are the two most common hostile types.
I never got much use out of Kneecapper (20% chance to cripple a leg), but I've seen some claim it's overpowered.
VATS enhanced (reduced AP cost) can be pretty good for pistols. It doesn't really help for weapons which take half your AP, since you're not getting a third shot with it, but it's pretty powerful with fast weapons. The archetype being the Deliverer, which is a very good weapon at short ranges.
Legendary armors - well, obviously useless if you're using power armor. Which is one of the problems I have with power armor, it makes the question of "which legendary armor do I wear and mod" moot.
If you're deliberately forgoing power armor, there's the problem a lot of legendary armor is pretty poor quality if you're ignoring the modifier. +2 to SPECIAL stats or 15% reduced damage isn't that great when it's on a basic Leather item, which it often is.