Nebulous
Legend
Summoned demons cannot summon more demons. It's one of those "learned from past mistakes" clarifications. So do it anyway. DM empowerment ftw.
Demons are more about the resistances. Yochlols cast web at will, cause incap in mist form, and can dominate once per day. A yochlol who summons another can dominate two party members, mistform, then incap two more. Then the dominated characters can do the damage for them.
The dominated victims don't need to be the PC's. An unfriendly NPC travelling with or parallel to the group works. Or a maguffin NPC the charaxters are trying to protect is now attacking them.
Work the controller aspect.
Oh, summoned demons can't summon? Now that you mention that, it does sound familiar, although I'm not sure if I read that in 5e or not. But yeah, you can fudge it as DM. Hell, maybe that's the yochol's "thing", it can keep summoning allies without failure. I would definitely consider that a "feat" to boost its power if that's what someone is looking for.
My yochol was on the wall holding a baby, and spamming the party with darkness and webs, but it did nothing. They all made the saves and the dark was only mildly annoying until they stepped out of it. The paladin saved against Domination, so the yochol's coolest ability of the day was wasted. If you want to add power, I would suggest bumping Domination to 3x day. It did not get the chance to summon another one though, the party retreated fast because a whole squad of drow were approaching and it would have been a fatal fight, I really don't think the party would have survived.