The dirty little secret with GIMP previews is that they are just approximations and indeed, not good ones. To use even G'MIC requires experience with those preset. As you know the behavior of the settings, you can then make an intelligent guess as to the outcome based on the preview. Trust me that it's not just G'MIC that has this preview issue w.r.t. GIMP plugins. Again, with experience, you get a mind's eye knowledge of what the output will be based on the inaccurate preview. Sorry about being the barer of bad news, but the truth is the truth.
