... works only with spherical/cubic objects, and produces bad looking transformations for any other shapes.
I am afraid this isn't so. Try and rotate your test.obj tires on two or three axes simultaneously -- there are no "bad looking transformations" in sight, and these meshes aren't spheroid or cuboid at all. Moreover, even your asymmetrical bolt meshes spin quite reliably around their centroids.
This kind of rotation was initiated by you, Patrice, not me. This is rotation around the mesh "center of mass", or centroid, and it is "axial" and more or less pleasing to the eye only when the mesh is generally orientated along one of the model's orthogonal axes.
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There is another kind of rotation -- around an arbitrarily inclined axis (-es), which requires a different type of matrix calc. But I think you still have a ton of meshes in the existing models that can be animated as-is to make them more attractive to the user. With time, when the #group syntax works and there are no more glitches, we could add axis rotation too, enriching our #rotate options accordingly.