I am accustomed to use custom normals rather then smoothing groups . Those groups rather complicate modelling imo. My geo and its shading look perfectly fine after import but each time I see that "no smoothing groups in fbx file" error. Should I care? Can it create some issues and smoothing groups are really necessary?
fbx supports maya style explicit normals and also max style smoothing group definitions - if you dont export with smoothing groups they dont get defined in the file, its not a false positive as such but it's entirely fine to ignore it
Maybe they are necessary for some advanced lighting baking or something? Distance field AO, Lightmass etc? So far I see no issues yet . Smoothing groups looks like kind of outdated concept for me so I wonder maybe I miss something? With custom normals I don't even have to triangulate meshes . Well, in our engine at least.