I personally think it’s a power user tool rather than an easy to use tool. The UI can feel intimidating but is actually pretty coherent once you understand how Syncthing works.
I would compare it with Git in terms of ergonomics : a powerful tool with its own jargon that you must understand to be able to use it.
Like git, Syncthing chose to expose its internals to the user rather than hiding it behind something magic. But like git, I don’t feel like there are unnecessary complexity. Once you understand it, it’s easy to make it work because it makes sense.
It does have GUI, which I use. I wouldn't call it pretty or polished, but it works and I understand how it works and the way it works is exactly how I think syncing should work.
I've also configured it to run a GUI diff tool diffuse to easily combine changes in case of conflicts (when a file was changed on both sides since the last sync).
I wish it was a bit more modern and re-written in a modern language, but that's secondary qualities for a program.