Summoning can be balanced but it does tend to break the game in one of a few ways
- being better tanks and meatbags than the characters built for such a purpose because the summons are expendable and generally take up more space.
- breaking the action economy (the help action is particularly relevant here, but reaction attacks as a means to control space are also relevant)
- exploiting aura buffs that were balanced with the assumption of only targeting 3-5 characters
- If you allow specific creatures, you'll always end up giving PCs access to abilities they definitely shouldn't have access to at a given level (conjure animals as a mass flying taxi service, pun-pun in 3.5, nonmagic bsp immunity, )
There's also a problem that mass summons in particular tend to be completely feast or famine. Either the enemy has no AOE and you run them over, or the dragon roasts all twenty of your zombies in the opening round at no extra cost.
Basically, its imo not worth the trouble. I know people like summons, but asking for more than the (very very strong) spells we have in Tasha's just feels whiny to me. But maybe I'm just a salty martial fanboy.