My ankles have gotten stronger for it! Learning to jump around on rocks, uneven surfaces, mud, roots, etc has really helped me strengthen my legs and ankles much more than before.
Here’s a quote from Cool Running that I found about running surfaces and their differences for your body:
Avoid rock-hard surfaces like concrete sidewalks and aim instead for grass or dirt trails. The idea, of course, is to run where the ground will absorb more shock, instead of passing it along to your legs. Try to be consistent. A sudden change to a new running surface can itself be a cause of injury. Whatever you do, though, get off the concrete. It’s approximately 10 times as hard as asphalt, and is easily the worst possible running surface. Runners World agrees, as evidenced by its ranking of surfaces on a scale of 1 (awful) to 10 (best):
Wood chips 9
Cinder track 7.5
I try to run a little bit of everything in a week. I just try to keep my longest runs on dirt/grass/gravel since that’s the worst pounding. My neighborhood has sidewalks and I do run those a bit, especially when the traffic is higher. I run alongside the road often too. Around my home, there’s about 5-7 feet of dirt/gravel alongside the 45 mph busy road that I run.
I do a little bit of running on asphalt too – since it mimics what I’ll be doing on race day (most of the time.) Usually I do my speedwork on asphalt or a track.