The only "it" in the previous short sentence was "balance"
Semantics.
You modified a class because it was less fun for you.
But you don't want to modify a class when it's less fun for others.
Also, I really don't see how indomitable makes it hard to design encounters. Seems far easier than...
You nerf fighters because you find it to be disruptive. But you don't want to modify necromancer even if others find it to be disruptive.
That's a double standard.
It's more fun when you are more powerful than the other PCs.
It's less fun when you are weaker than them.
So balance is important.
Of course, you can have 18 skeletons and still be weak.
I.e. any damage your skeleton takes, you take instead. And they only have 10 AC and vulnerable to...
Just to toss out more ideas.
Limit in combat actions by player actions.
I.e. player uses their bonus action to move a skeleton, action to make it attack, reaction to make an opportunity attacks.
Scaling to commanding 4 at once.
Commanding them takes concentration, but they will just stand...
IMO, id your doing a full class, make it a generic Summoner class. You're going to need nearly all the same mechanics if your summoning undead horde or elemental swarm.
IMO, the spell is stronger if you have bodies.
You don't want a campaign on a deserted island to completely remove all the features.
Side question. Why human skeletons? Seems like animating undead ants would be easier.
Just to bullet point things I would want in an undead swarm
Grapple
Difficult terrain
Moves slowly, not slowed by Grapple
Low but unavoidable damage
Size is based on HP/damage/spell slot
Grows/heals over time, possibly as an action.
Bonus HP/size if you include a corpse
Experimental thinking...
Seems perfectly reasonable for a DM to allow Summon Undead Horde to push a rock. Same as any other summon.
Which is probably a good reason to use a stat block instead of a zone that I initially suggested.
You got 1 skeleton by level 1 (which is less than 3rd). And your army grows every 2/3 levels after that.
That was the post I quoted.
Didn't see a way to get 10, but there's a few levels left.
Which is great if you don't have too many battles in a day.
Sorcerer's are best against a few powerful enemies. Not so good against 6 battles a day containing 5 easy enemies each.