If a developer is already an expert in the back end using Go, then great. Maybe they’ll bring a different perspective when they work on the front end. And they will probably enjoy a new challenge.
Web development is not so hard that a good developer can’t learn the basics in a couple months, especially with some mentoring.
There are so many things and concepts, multiple must-know languages, browser quirks, some networking knowledge, CORS, etc. If you do use an "industry-strength" backend framework then the complexity surely drops, e.g. it will handle injections and stuff, but not having heard at least a bit about what your framework does for you, and reinventing the wheel can go really bad really fast.
But I think it’s a sign of an unhealthy or immature team if it is unable to onboard and train new developers.
You are right that a new developer could easily make a lot of mistakes, but it is the responsibility of the senior developers on the team to give feedback and review the code of less experienced members.