All but one of my snakes are on f/t and I haven't had any problems. I don't believe a snake who's on live is more healthy/alert/happy than one who's on f/t: as long as they're eating they'll have energy, be alert to look for their next meal, and be as happy as a snake can be as long as husbandry is on too.
My only problems is that some of my snakes will take the feeder then not eat it, or miss when striking and then refuse. Though those can be issues with feeding live too: another downside to f/t is that I don't refreeze the feeders (preference) if I have a refusal so it can be a waste of money.