People will always drag out a draw power pokemon and kill it if they can. That's been big since Claydol; it's just too much of a threat to ignore. Now some may argue that Magnezone can defend itself, and that is certainly true. The problem is that a deck that isn't built around Lost Burn can find itself in a lot of trouble if it actually uses the attack.
More importantly is Catcher, not on Magnezone per se, but on Magnemite. Magnezone is pretty beefy and an opponent needs to use a fair amount of resources to kill it, instead of a main attacker like you said, but Magnemite is fairly easy to knock out, and rare candy rules mean that Magnemite will be vulnerable for one turn. Magnzone getting dragged out isn't as big a deal if you have switch handy, but Magnemite will get KOed very easily and you need to watch out for that.