I'm not sure the legendary items would be items from D&D per se, it seems Larian likes to tinker with them quite a bit to synergize with their interpretations of class strengths and weaknesses, though I am welcome to eat my words. I think the amount of them is not really concerning, as it is really Larian just trying to open up avenues of play and distinguishing the power curves of classes a bit more, after all, like Tuco mentioned, BG2 was not stingy with these things by any means.
It was throwing +3 weapons at you very quickly, not to mention other items of ridiculous power relatively early in the game. Given they've likely balanced the difficulty of subsequent battles within the confines of characters equipping these kinds of items, it would seem to be a bit foolish not to utilize them.