I'll leave with this. ANY service exposed publicly or not should not have vulnerabilities. If there is any hint that your NAS webserver has vulnerabilities, it shouldn't even be used internally. So to me, it does not matter. I don't expose my NAS webserver because I have no reason to increase my attack surface that wide.

But I'm comfortable exposing any of my internal services as needed because I've personally checked the source code for vulnerabilities, and have proper checks in place on top of regular security updates. I understand why others wouldn't think the same way, as this takes a high level of confidence in your ability to assess the security posture of your systems and network. I've had penetration tests in my network, conduct them myself for business.