So Undead Thralls adds Animate Dead to your spellbook if it isn't already there, at level 6. You can get a free spell by holding off on Animate Dead until level 6, but why would the Necromancy school of all schools encourage a Wizard to use this spell later than all other classes? Or you take it at level 5 like any other Wizard could, and miss out on the free addition to your spellbook.
Both of those seem more like a punishment than a benefit. I was considering homebrewing this to grant an additional Necromancy spell (of level 3 or lower), but I want to make sure I'm not missing some strong benefit of this first.