Car pushing/pulling wouldn't be a great replacement for leg work in the gym, but would be fine to do in addition for something extra. As I've seen, the greatest factor for what makes it "difficult" lies most in the need to have a very tiny incline, as unless you have someone put a bit of pressure on the brake, a moving car tends to keep moving fairly easily. I know this from experience - some of us had a small impromptu strength event some years back, and the women wanted to push the car as well. A small/medium sized Ford Focus on perfectly flat blacktop was easy as heck for them to push, none of them trained any sort of athletics, and they all said it was easier than they thought. Not that they weren't a bit tired from it, but without making it extra-difficult, it is more of short but intense cardio workout, as if you did near max resistance on a bike for 30 seconds. Sure, your legs might hurt a bit, but it won't really do much to make them stronger. But, as far as working things in new ways and building up some added strength endurance, pushing/pulling cars can be good, just be safe when you do it!
"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous