I really think the only thing lacking is the cab modeling; if they could use larger IR's or a more robust algorithm (still have no idea how the cabs are done by L6) somethings would sound a lot closer.
I always remind people when they ask about sounds "This thing is modeling the sound of your head (microphone) few inches away from a speaker...if you want your headphones to sound like an amp in the room, crank up the ER in cab block and cut some highs. If I could afford all the stuff thats in the HD in analog, I still woudln't, cause it would literally be a truck full of lollipop just for my guitar gear. Now I have two guitar cases, pedal bag, Power Block and cab, and I'm good to go. Never had anyone come up to me after a show and be like "I could hear that you were using the L6 HD500" so it must not be "off" in a bad way, right?