Yeah. I imagine it would be similar to if they were to implement drivable vehicles into Fallout. It opens the world up to more technical issues like collision mechanics (what happens if you hit a large object that's normally not movable but should be by a giant mech/vehicle) and making sure some areas are fine for them (not clipping through decorative items, squishing things, clipping out of bounds, and so on) and others are restricted in a way to prevent their entry.