Single summoning limit is omething that is around since nwn2 probably something that was put in for balancing. But was not needed in bg npc could summon as well creating interesting battle scenarious.
I don't even know why this limit is retained to start with.
1 - In a SP game, FUN > Balance.
2 - A lot of D&D games din't had this BS
3 - Consistency in mechanics and lore >>>> balance. And a necromancer unable to control two undeads makes ZERO sense.
4 - Balance is subjective. Some people complain about bolt action rifles and shotguns on BF1, literally the less used weapons in the game and the weapons which people should use the most in a WW1 scenario.
5 - D&D was never balanced, except 4e which is the worst edition by far.
And even 4e, WoTC prefered to not include necromancy for the sake of the balance dogma than including a USELESS SINGLE summon limit on 4e. 4e was awful but at least they din't included an SUB class that serves to nothing but to cause frustration among the class fans.