Just checked the template for myself; here's the relevant info:

Thrall to Thoon Infiltrator: A Thoon thrall responds to the commands of the nearest Thoon infiltrator as if dominated, with no save allowed.
Overdrive Healing (Su): This ability works like fast healing 5, but the thrall can gain hit points beyond its full normal hit points as temporary hit points. The Thoon thrall’s skin begins to blister and swell when this happens, and the thrall grows visibly larger. Once its temporary hit points equal or exceed its full normal hit points, the Thoon thrall must make a save every round or explode (see below).
Immolate (Su): When a Thoon thrall is fully healed and has temporary hit points equal to or greater than its full normal hit points, it might explode, its body unable to contain the energy within. At the end of its turn, if a Thoon thrall’s temporary hit points equal or exceed its full normal hit points, it must succeed on a Fortitude save (DC 10 + 1/2 the Thoon thrall’s HD + its Con modifier).
So (A) they act as if dominated, which doesn't necessarily mean immunity to [mind-affecting] effects would help, (B) the temporary hit points are based on fast healing, which undead can possess, as opposed to regeneration or Positive Energy Plane shenanigans, which they can't, and (C) the Fort save DC is fixed. Based on A and C, I'd say becoming undead isn't really worth it, as a positive Con modifier helps with the save.