It's faithful enough to 5e that my partner and I broke out the players handbook to do some long term class planning together. A couple of things are different, like buffs to frenzy barbarian and changes to roleplay feats or spells to have a more mechanical benefit.
But yes, as a long term DM for 5e, it's faithful to 5e.
I know that Mugen is just a word, but to me and a lot of other people, Mugen is the build your fighting game engine with a constant twitch stream that has Goku fighting and losing to Tetris brick.