The main reason why your tend to pick up less chronic/over-use injuries running off-road is that, because of the constantly varying terrain, every foot-strike is different. Repetitively jarring your heel into the ground in exactly the same way stride after stride just isn't possible. Also because fell shoes tend to be much more minimal in construction, your feet can work much more naturally and you'll find yourself adapting a mid/fore-foot strike without really thinking about it. Don't worry too much about "downhill stress on the knees", it's really a bit of a myth... the more technical/steeper the downhill, the more lightly and nimbly you're forced to run.... Just get out on those hills and enjoy it!