1) The main difference is in their approach to security. OpenBSD's primary focus is a complete, open source, BSD system with a focus on security. The way they attempt to and often succeed at achieving that goal is through "code correctness" and emphasizing the use of integrated cryptography. That isn't to say FreeBSD is insecure, it just isn't their primary focus.

2) I'll let Theo answer that... http://marc.info/?l=openbsd-misc&m=132788027403910&w=2

3) Not "big issues". But since OpenBSD doesn't sign NDAs to get firmware or drivers, and since they like to rework any software that isn't up to their standards, they do lag behind in hardware support. A good way to check if what you have is supported is to read their release notes. For example https://www.openbsd.org/60.html has a list under "improved hardware support".