One issue is that you don't write Go code, you write Go plus some templating language (like html/template or go templ). Not being able to seamlessly move from regular code and template code adds friction and is limiting while developing, figuring stuff out and iterating.

Another problem is that the type system is often not expressive enough for frontend code. So you either have to generate types or end up with something like map[string]any, which is awkward and gives you less guarantees and support than what dynamic languages typically offer (in other words, dynamic languages are better at being dynamic languages than Go).

Now these problems don't emerge in every web UI based project, but when they do, it's really painful and limiting compared to what one is used to.