While the Hugo developers are very responsive and do a good job maintaining the project, they don't generally respect their users (be they theme or site developers) time. Every few versions I find that my sites are broken because the developers changed something on me, sometimes in ways that just silently change the behavior of the site (as opposed to a build error, which would be annoying but which is more discoverable).
The documentation is also very difficult for me to learn from. Despite being extensive, it's extremely hard to learn how to do anything and I find that it frequently makes assumptions that you already are an expert theme or site developer. I wish I had a better grasp on what makes their documentation so hard to use despite the fact that there's so much of it, but I find that what I'm reading is almost never what I need to actually understand what's going on. This might just be me though.