Is there a risk in opening port 22 for ssh access with a network?

Can someone clearly highlight why this is not recommended?

Because any running service is increasing your attack surface. Especially with network capable services, you're always exposed to danger. I don't think anyone can give you a more specific answer than that, since the SSH implementations tend to be quite OK from a security standpoint. Nevertheless there is always the possibility that an exploit is discovered, granting anyone sending the right packets to your server access or that you've greatly misconfigured your servers.

But, even if you assume that the software itself is flawless, there are a couple of reasons why you might want to re-think your approach. Obviously, you haven't told us and details about your setup, but in the real world key management, audit trails and firewalling is always an issue. Aiming for another approach might therefore be a good idea.