Maybe it's not impossible but just quite difficult? I use Houdini 'Vops' sometimes and I could imagine a tricked-up diff could be made for it (especially since

Maybe it's not impossible but just quite difficult? I use Houdini 'Vops' sometimes and I could imagine a tricked-up diff could be made for it (especially since it translates to vex) but you're certainly right that it's a hard problem in general!