I’ve not used a manual treadmill except on a couple of occasions to try them out for novelty’s sake. I think they will have a learning curve but should work fine once you figure out how to run or hike on them. However, If you plan to train a lot like this for mountain pursuits you might consider an incline trainer. These can go up to 40%. They are brutal but VERY effective.
Either way if you don’t have good hills to train on near home a machine like these will enhance your preparation.