(Disclaimer: I am the author of dear imgui)
I feel that Promit's answer is just rehashing a common answer that has been going round and people keep repeating because this is what their instinct tells them, rather than what they experienced directly. There is a bit of truth to it but it's also a very unbalanced and very misleading answer ihmo.
There are many factors at play, but if anything there are more than enough references in the Dear ImGui Galleries to suggest that it is perfectly fine for the use you intend to do with it. In fact Unity3d is built upon their own IMGUI implementation which it multi-pass and tends to be slower than a single-pass UI. Please see if you can find many actual users of Dear ImGui complaining about performances or scalability issues.
The limiting factors are more about to do with the maturity of a specific framework. Dear ImGui is evidently less feature-complete and less mature than established RMGUI frameworks. But the productivity/flexibility boost can also give you wings and make you create things you wouldn't otherwise.
There's not a single answer. Even the IMGUI/RMGUI dichotomy is largely misleading, where are plenty of factors we intuitively associated to one side (because of existing impl) which have nothing to do with the immediate-mode-api-ness. We are just lacking the choices and R&D to push us forward (even Dear ImGui is miles behind what I wish it was, for lack of appropriate dev team)
I would say go for what you are confortable with.