What makes Melee so difficult is not necessarily the individual techniques, since being able to perform them in isolation is pretty attainable for most players, but rather the fact that all of the tech has to be dynamically applied in action chains.
Although the individual actions and some action chains are obviously commuted to muscle memory, a lot of the tech skill you're going to apply has to be decided on the fly in extremely narrow windows of time, which can often lead to technical flubs in a chain of actions. So while performing that chain would be easy in solo practice when you're prepared to perform every action, the dynamic nature of a match changes things completely.
To me top level Melee play isn't so much defined by technical skill as it is incredible presence of mind and decision making speed. Some players may compensate for the lack of the latter with great knowledge of options within the game and planning everything several steps in advance, but then that ties to the former, having the presence of mind to assess and understand the many options within the game before committing to a sequence of actions.
This level of depth, pace and flat out difficulty makes Melee rightfully daunting to tackle but hey it's also makes it so rewarding and fun to play, as well as to watch.
I can't compare it to MvC, but as a game in general, Melee has a level of strategic depth combined with execution requirements that make me feel the game has one of the highest skill ceilings of anything (even outside of just videoganes) I've ever seen.
But idk just my .02
