I know that at least one modding framework for a certain online game makes use of ImGui (or some equivalent thereof). Given the use case it does make a lot of sense, considering they're essentially strapping a third party UI onto an existing 3D accelerated application, not sure what else you'd use for that. Since the users are technical enough to install the mod framework anyways, they tend to be the sort that can handle the UI.

It can be a bit wonky though, I regularly spot UI/UX decisions that seem to map more closely to what the developer is doing under the hood, or their own mental model of the problem, than what one might consider to be an intuitive way of interacting with the system.