Carts get all kinds of fucked up once you get onto a horse
#1 If you are on the horse's crawl speed the cart will derp into the back of the horse and then detach itself from your character (you might be able to walk for a bit, but within a few clicks you'll block yourself)
#2 If you go up to walk speed on the horse, the cart is unable to keep up with you regardless of terrain
#3 If you are on a horse and pulling a wagon and then you pick up an object overhead, the cart switches to crawl speed