Which actually gets me thinking. Slow works against Undead too, right?
Indeed.
Trouble is its difficult to get Transmutation DCs high.
As an alternative for Druids/Shamans there's also Conjuration Slowing Mud - Fortitude based.
Not that Conjuration DCs are that much better.
In act1/2 there isn't a really big difference in DC. Only DC items you get is a +1 will DC staff which works on slow, a +1 evocation DC dagger and a +2 illusion ring. The illusion ring is pretty whatever for most of act 2 (rainbow pattern is pretty good on gargoyles though). Undead is mostly an act 2 problem so transmutation or conjuration should pretty much be at full power there regardless of how they are itemized later. The difference becomes big when you open the drezen vendors and then Mind affecting spells skyrocket ahead of the rest (although I do not know what IE and Treasure DLC bring item wise since I do not own them)
And at worse there is expanded arsenal, even without the bug, this feat is massive - you can tech for the early game and transition in midgame to a better school. I find slow/conjurations the be the best options for most of act1/2, even though they are the least supported schools item wise later. In vanilla selective grease is a no brainer, and in TTT I find slow to be the most reliable. So I like taking focuses in conjuration or transmutation early game and going for expanded arsenal at M2 delaying Spell focus mythic for M4 which I rush in act3. Its not like those feats are wasted either because in a pinch you might want to target a weak reflex save or something, and with debuffs even weaker DCs can go through saves. And what I have noticed is that baleful polymorph isn't immuned by many things which makes it a good fall back when Mind affecting spells won't do the trick. It is also better supported than the rest of transmutation because it targets fortitude and there is at least one item that boost fort save dc.
