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.
Best Answer
The meat of Undead Thralls is the enhancement to Animate Dead, and it's a highly effective boost, especially when you have the spell slot headroom to upcast and can have large numbers of undead at hand. Awarding the spell for free is more of a ribbon, rewarding patience, than anything else. It's not terribly potent, because Wizards can eventually add every class spell in the game to their spellbook, anyway.
The delay is an unfortunate side effect of the way the Wizard (and many other) classes are structured - they gain access to L3 spells at character level 5, and then an Arcane Tradition feature at L6. Giving the Necromancer at bonus spell at character level 5 simply wouldn't fit the established pattern.
You could certainly let at Necromancer pick a different spell without it affecting balance in any significant way. They get one extra free spell as compared to other wizards either way.