I am not aware of any other domain registrar that blocks VPNs, so the fact that Namecheap does is a drawback that is particular to Namecheap.

It's inconvenient to turn off a VPN just to use Namecheap. Although Namecheap already has my information, if I have more than one browser tab open and need to do something on another site while interacting with Namecheap, I would be exposed to an unprotected connection with not only Namecheap but also the other site. This is unacceptable considering that other registrars don't have anti-VPN restrictions.