One is Go's module/import system. It's pretty inflexible and the connection between exported functions and types and how they are Capitalized or not does create friction for me when I'm "building up" code bases from scratch. Puzzling about module boundaries and type definitions feels like it would consume more time than it's worth in a UI oriented program.
The other is most UIs are wrappers around other functionality, often in libraries. Working on anything other than pure Go code bases adds additional friction, and dynamic libraries add to that complication considerably.
On the other hand, Go's defining feature: Its CSP model, lifted from Newsqueak, was originally designed for user interfaces.
I believe Go could be a really great fit for UIs, but likely never will be as there is little economic incentive to put in the incredible heavy lifting required to build up the support libraries. There are certainly some small projects trying, but without massive resources backing it, it is a struggle to achieve the full fit and finish that we've come to expect.
The other is most UIs are wrappers around other functionality, often in libraries. Working on anything other than pure Go code bases adds additional friction, and dynamic libraries add to that complication considerably.