Mortal wounds = no saves, no exceptions (except for probably someone has a stupid special rule) And yes, all mortal wounds are cheap if you don't have a points cost, but there is a cost to losing large numbers of models or of only fielding a small number (and then summoning) and losing a few models if there is a minor victory to be had. If you field a big army there is a risk your opponent will get to choose a sudden death option. Anyhow here find some Math AoS on how to get the most mortal wounds per model. I will serve: Jungle swarms. 0.27 MW per combat round per model (5+ to hit x 6+ to MW x 5 attacks) Threat range 6 inches + ~7' charge. Can your model beat swarmy? The ball is in your court.
Har Ganeth Executioners, 3+ to hit 6+ to MW x 2 attacks, (3 for Draich) If a Executioner rolls 6+ the Draich inflicts 2 mortal wounds does that beat swampy? (I'm crap at maths)
Another thing to consider is forcing Battleshock on low Bravery units... if you affect your opponent's Bravery enough, you're getting free mortal wounds. That's especially valuable on multi-wound models because they're removed from play instantly. Troglodon parks outside of a combat... Primeval Roar passive, -1 Bravery... Saurus are in the fight... Stardrake Icon, -1 Bravery... Troglodon's spit does pretty nice damage now (especially buffed by a Skink Chief), and Saurus units put on a beating in close combat. Get a decent number of kills, force that passive -2 Bravery on your opponent plus whatever models were slain, and they're gonna be having a lot of problems with unsaveable wounds. If they have 20 models to negate your -2 passive, that's good for them.... but they're not gaining that +2 either. Kill one Mournfang, and it's taking a Battleshock test on 4 Bravery. Not good for them - on a 4+, they lose a model with 6 wounds (unless there are more than 10, which would be crazy). Now, that's Mournfangs. It's tough to bring down something with 6 wounds. Do this to Chaos Warriors or any of the many other 2- or 3-wound unit choices, and you can bet that a lot of them will be fleeing every phase. Tetto would be brutal here -- if you do well on the insight thing (or even if you get just one die), you can force your opponent to re-roll their Battleshock dice. Combo that with the Curse of Fates in order to further increase their Battleshock roll.... ouch. Of course, the general can counter this with inspiring presence if he's within 12" and you didn't go first... but that sets you up for a very nice feint. Goodnight, General! Some combination of mortal wounds + constant battleshocks in your favor is lethal. Bloodroar looks effective, especially with Tetto. Tide of Snakes, maybe? Rippers? Heavy Cargo? Arcane Bolt? Swarms? Very large volumes of attacks? Plenty of options...
I agree mortal wounds may be a bit cheap. However, it seems every army has access to deal them in some manner and our bastiladons have a 4+ "ward" against them as well.
Plus, you know the whole ability to deal up to 12 mortal wounds themselves. I honestly think that Basti's with an Ark are going to be amazing at clearing large mobs.