Pydantic is not just a toy, I built it having used and abused numerous other libraries and found them wanting in one way or another.

Because it reuses python's typing system it should have the most pythonic and flexible description of types possible.

I agree about the need for complex validation chains relating to numerous fields, that's already partially possible with pydantic (although not documented). I'll add support for this stuff as well as documentation over the next few weeks.