With 30 recruits, this card deals 30 damage. M is always rounded up to a whole value. With three Rare Undead in hand, it deals double the number of recruits in damage.
I disagree with your assessment. I've won more than a couple games with this card, and it's one where the player needs to make careful calculations to play it optimally. With this in hand, the best thing to do is to look and see what cards you have that give a worthwhile damage/recruits ratio, taking the keyword effect into account, to see how quickly you can make it a deathblow. Simply discarding until it's enough can give your opponent too much time to prepare or get a lucky draw. And you need it to be your final play (or make sure you have a cheap card to deploy afterward), because if you're playing Undead, having only your Dungeon's worth of recruits afterward can leave you "dead" on the water.
I also disagree that having more than one Rare Undead in hand is an unlikely event. If you have this card in hand, you're going to consider using
Necromancy amplifier and
Necromancy and
Lich specifically to increase those numbers, if your deck has them. (Mine has the latter two.)
Crypt also contributes well toward this since you have a good chance of increasing your recruits with it while possibly adding another Rare. Getting three or more Rare Undead in hand can certainly be done on purpose.