nginx restart failed is an error that should not happen and be debugged as soon as possible, nonetheless HestiaCP should react gracefully to it, especially in cases like a web domain creation where it is not essential (unlike when creating an SSL Certificate afaik).
When I create a Web Domain and check the boxes to create a DNS and Mail Domain with it, and nginx restart fails, only the Web domain is created, leading to a non-functional domain which could be especially confusing for customers.