SIT_FLAG_NO_COLLIDE (applied with llSetLinkSitFlags()) disables collision shapes on seated avatars but avatar mass is still contributed to the object. This shifts the physical object's center of mass and interferes with the function of particularly small vehicles.
Avatar mass contributions should be simply be disabled entirely when SIT_FLAG_NO_COLLIDE is applied to match the behavior of linked objects not contributing to a linkset's mass when PRIM_PHYSICS_SHAPE_TYPE is set to PRIM_PHYSICS_SHAPE_NONE with llSetLinkPrimitiveParams().
Alternatively, a sit flag, e.g. 'SIT_FLAGS_NO_MASS', that disables mass contribution from seated avatars is proposed.