So, if someone went medic, and bought the be the hhh, they would regenerate health. This would be fine, but because hhh takes much less damage enemies wouldn't be able to do anything to stop it. Even if they did, all you have to do is spook or kill them and then go hide. It takes a full in your face brass beast crit clip to drop the hhh when restored to full by regen. This kind of damage isn't exactly possible in normal play.
I think a proper fix would be make the horseman take regular damage, but have his usual high hp (3000 +200 for each player in the server is what the wiki states) and upon purchase player would refill hp. That way the regen wouldn't out heal the damage caused. As it is now, crockets do less than 20 hp, and that is quickly regenerated by the medic.