i've never used hardboots on a snowboard, just for mountaineering - but the definitely have their place on a splitboard for some people.
big advantage is going to be obviously touring. less weight on your feet, more lateral stability for traversing, better for kicking steps, wearing crampons, spending the night in the snow and other manly mountaineering type of activities.
disadvantages might be that you're higher up off the board (is this even true?), cost of the system, and the (possibly?) inability to bring as much freestyle into how you ride because now you're robocop and robocop don't ride duck.