think a very big problem is, as has already been mentioned, tacticals are the only unit that really gets a good plasma weapon. So tacticals are a great counter to tacticals (and ASMs for mirror...grrr), and that's about it.
Everything else has to get to within melee range, which by then, is advantage: tacticals.
Unless you also have a disruption unit in there (stikkbommas, rangers), but because of that you have fewer power melee squads to tie up and efficiently damage tacticals with, and that means advantage: tacticals.
Also, I believe power weapons do 1.3 and plasma 1.5 to heavy armor, and we might be seeing the result of that. Even if the power melee closes, after they run through the plasma guns, all other things being equal, it's still advantage:tacticals.