Yes, shots 1,2,3, and 4 have a decent background for panning: stands with spectators in them. Shots 5 and 6 have a bad backdrop, with the portable awning and the parked cars and the building with the many repeating white panels over dark green paint...that background steals a lot of thunder from the vehicles being panned. Centering a panned vehicle or bike in the frame is one of my pet peeves. The vehicle always needs more space "to drive into", so it's almost always best to leave a good deal more space in front of the front bumper!!! The SMALLER space behind the read bumper shows the car as "moving".....where it "was", and mentally, the greater space in FRONT is the space that the vehicle will soon be "driving into".
For me, panning is one of the PERFECT examples of where using a LEFT- or RIGHT-side-of-frame, multi-point AF arrangement makes the most sense. On many cameras, not all, but many, the outer AF points are NOT cross-type sensors and are therefore not always that sensitive compared to a cross-type sensor. But, by using a multi-area AF arrangement, like say 9- or 11 AF points in various Nikons, and shifting the active AF cluster off to the side where the panned subject is, the camera can almost always get a focus lock, and can track the subject.