A few days ago I posted on social media about flat v hills. This is an important component of training specificity. So in general, for optimal performance and reduced injury risk, a large component of your training should relate to how you are going to compete. Training 100% on an arena or all-weather track will not fully prepare your horse to compete on grass. Training long-distance-slow will not prepare your horse for sprinting. Training in the cold will not prepare your horse for competing in the heat.
Terrain is also one of these important considerations if you are competing in competitions where the terrain (hills) may vary i.e. eventing, endurance, driving, point-2-point, etc. Training on the flat induces different muscle recruitment patterns. Training on the flat usually results in longer stride length and lower stride frequency; training on hills results in shorter stride length and high stride frequency.
This is not saying if you train over hills you CANNOT compete on the flat, but this will not be optimal. If you only have flat ground to train on then choose relatively flat competitions – for optimal performance and reduced injury risk!
If you train on a mixture of hills and flat then provided you have a balance that is close to the competition conditions, then this will be better than only flat or only hill training.