I also used a yochol for the first time ever recently in any edition. It was a nanny for a drow prince/noble, which is weird, but the imagery was too cool to pass up. I had it crawl up a wall far out of melee range and spammed some webs and darkness on the party, which was mildly annoying at best. They didn't do much damage but managed to kidnap the baby and hold it ransom in front of the kid's mother and father, threatening to kill it if they didn't let the party go. In truth, I think it was going to be a TPK if the PCs didn't extradite themselves from the situation, as they were about to get pounded by five drow and a yochol and giant spider (and maybe a phase spider from the ethereal).
Was the yochol underpowered? Eh, slightly yes. Toe to toe against the whole party in a round it would probably die (they were 5th level). Now, like
@vincegetorix said, summoning another yochol would DRASTICALLY change the encounter. As DM you can even fudge that 50% thing to 100%. Every yochol teleporting in and immediately summoning another yochol would turn this a nearly unbeatable scenario. And not to mention, it's poison damage slam attack is quite deadly, critting with that could one shot some heroes.
I highly suggest looking at the Expanded Monster Manual, it takes about every monster in the core MM and upgrades them to new types. The Yochol Elder is CR 13, and has a bevy of new attacks and defenses and is horrifying.
https://i.*****.com/28Hqk6U.jpg