I have no problem whatsoever with the character concept, at its root. An fellow N7 operative, an elite commando sharing much of shepard'ining and background, but his racism descends into extremism. Unlike the conflicted killers that make up many of the squadies, he's a actual sociopath. Not a hollywood cliche sociopath, but to paraphrase the movie Tombstone "A man like that has an empty hole in the middle of him, no matter what he does he can't ever fill it. He can't kill enough or steal enough or inflict enough pain to ever fill it" Follows TIM without question, and has a bunch of reaper tech upgrades. That's the making of a worthy villain. Here's what went wrong:
1: His appearance is like a corporate think tank trying to come up with something that appeals to anime fans by committee.
2: They establish his badassery through invoking the Worf Effect trope on thane, and having characters who have never mentioned him all agreeing that he is SO dangerous. Yet he fails to perform up to the hype.
3: Mocking a dead companion, and the PC in combat is about as subtle and original as the dying kid in the beginning.
Man had potential, but they tried to hard to make him cool and edgy while ignoring ME design themes and feel, and got lazy and relied on common tropes in key scenes.
As far as the phantoms go, get rid of the ridiculous space katana, give them an omniblade attack, and they'd be perfect.