Oxalate crystals, I think. I've had a few panaqolus chew on pothos rhizome and occasionally roots, but they would only do that on occasion.
Oddly enough, some of my past turtles and tortoises would eat pothos with great relish if they ever got a chance- which was almost never, as each time they were able to sample the stuff was due to accident or negligence on my part.
On a random note, when I was a young kid, I was curious as to how they'd taste, and ate a couple leaves; they're oddly reminiscent of weird, smooth spinach that's a bit tougher to chew. They didn't taste all that bad, I think.
Monstera adansonii, on the other hand, was similar to very thin, bitter, almost powdery celery/bean sprouts (?) with spinach-like leaves as well.
Probably not a good decision to sample them, in hindsight.