I'm no expert but in my experience they can do fine with hills. You just need to be sure that they're conditioned for it and that they are physically capable of doing so. I worked a couple of minis in a cart going up and down hills, they did just fine.

 

there is a driving section you might post in but a lot of it would depend on how much you weigh and your cart weighs. It can be a struggle if you are heavy. Another big concern is not going up but down the hill. If you and the cart weigh much you will be pushing them down the hill and possibly off there feet.

 

A horse can pull up to 5 times it's weight. When I would condition the driving show horses, once they were seasoned enough that I trusted them outside the arena, I would drive them on the hay road. We won more at the AMHR Nationals that year than ever before because the horses were in such great shape. Working up to the hills are very important. You need to take into consideration the overall fitness of the animal and pay attention to how they carry themselves when they aren't strained so that if the norm changes you know you are pushing too far. Deep footing is much more of a concern as far as pulling stifles and injuries like that. Driving is a blast! I prefer to drive 32''-38'' as they have more power than ones under 32''.

 

Good info so far. Also remember that if you're dealing with hills its even more important for your harness/rigging and cart to fit the mini correctly.

 

Make sure your harness has breeching. Some of the show harnesses do not, but if you are working on hills you will need it.

We have minis we are training to drive and we have hills. We were told that minis can usually pull about 3X their weight which is usually 2 adults in a regular easy-entry cart. With the hills, we are keeping it just 1 person per cart and slowly working up the conditioning.