I don't actually know if mindless units dissolve without leaderships. Most mindless i can remember right now are also magic or undead.
Lifeless is "undeath light". They're immune to a lot of spells that need some kind of metabolism to work. Poison, disease and miasma effects are three examples.
There are some spells that specifically target one of those three categories (like the all-popular anti-undead spells) or do specifically not target them (like mindblast and mindless units).