Except for possibly against confusion Mewtwo, I don't see how alakazam would ever be a better Shadow attacker than hex gengar.
Confusion Mewtwo may kill gengar with only fast attacks, but other raid bosses should all need a charge attack to get the kill - so it doesn't help alakazam to get to that charge move with 55% health instead of 20%.
It's usable of course, and probably there exist cases where it's better than gengar in mewtwo confusion raids depending on when it uses the charge move, but otherwise not optimal.