The main problem with Natural Spell (in my opinion) is that while it works for Shifter (PnP), it doesn't work for Druids (they aren't even able to take the feat)...now, I'm sure that in PnP, druids can take and benefit from Natural Spell, however, currently regular ol' Wild Shape/Elemental Shape/etc (non-'Shifter (PnP)' shifting, in other words) use Bioware's polymorph.
So I don't know if Natural Spell would even work with Bioware's Polymorph system. If it doesn't, is it in the plans at some point in the future to convert druids to the PRC shifting system as well?
Also, regarding Warlocks qualifying for Shifters, perhaps that could be made optional via a switch in the PRC options? Just a suggestion incase some folks might feel that invocations should count as 'spellcasting'. (The switches are, after all, meant for 'house rules', right? i.e. "we'd rather have 3.0 Heal" etc)