Is there any problem in being both a user and a pub in the same sbot instance/process/id/keypair?

nope, That is perfectly fine. A pub is no different from other sbot servers - it's just reachable _pub_licly.

I've posted pub messages about myself with onion hosts in the past and other have as well. Same works for other vpns or overlay networks like cjdns.

The here be dragons territory starts when you copy the .ssb/secret to more than one machine and worse: post from multiple instances.