Also, for a roughly apples to apples comparison, wall of bones has 12 life, zero armor. Reassemble is a 4 mana quick cast that heals 6 life on average. A growth counter on a resilient creature is similar to 6 life on an unarmored creature statistically. Level 1 zombies cost 4 for one potential growth, 2's cost 8 on average for 2 growth, and 11 for 3 growth. Seems in line number wise to me with the existing wall of bones.
For using a full action and a slower delivery method, the zombie wall gets a counter strike as well.
In terms of sacrificing a level three or equivalent combination, yes it's hard to take down, but the necromancer already has easily healed walls and the warlord has the steel wall which he has the ability to repair. Maybe I'm wrong, but I didn't think it outshone those options. At that point, it's similar to a level 4 wall with 28 life and no armor for 14-17 mana. 14 if you sacrifice a legendary creature to it. It costs 6 for a 12 life no armor wall of bones.
Edit: fixed math on level 2's. Added commentary at bottom.