Trails can be really beautiful and fun to run on—not at altitude for me, thank you very much, unless you're acclimated to it and used to living there—and they have a way of making the miles pass faster than you think ? until you look down at your watch and realize that, thanks to that climb, the miles were technically pretty slow!
I've never lived in a place where there were ample trails that were safe to run on year round. You need to be really careful because if you're not you'll wind up hurt, or worse. World-class trail runner Michael Wardian said, "I remind myself not to zone out while outside and especially on the trails where a bad footfall can mean stitches and a new tooth."
When to Dodge the Trails
Really rocky: One of the benefits of trail running is that it can be a much more forgiving surface than concrete IF it's actually soft terrain. On some trails, you can run on rocks and gravel, which is not going to give your legs anything in the way of cushioning. On top of that, the loose gravel stuff could set you up for a nasty fall or ankle twist.
Slip and sliding on slick trails: In Oregon, where I used to live, there are some beautiful trails but, hello, it's Oregon and it rains a ton, which means that the trails were really slick and slippery the vast majority of the time. If there is a lot of foliage, you'll slip and slide on a bunch of wet leaves.
Drastic uphill and downhill running: Lots of uphill running will make you sore. This can be a good thing if you're looking for some harder work to build strength, but tons of downhill running can take a toll. The additional pounding and force of each footfall when running downhill is tremendous; it's really hard on your quads, knees and joints. Be careful if your route has a lot of downhill.
Roots, twists, cougars, darkness and the other stuff: Since we're discussing safety, it's important to bring up the obvious factors here. You need to practice running safety regardless of where you're going, but if you're going solo for a trail run, be sure to tell people where you're going and how long you plan to be out. No one wants to go out for a run and end up having a search party bring them home.
That said, there are a myriad of benefits to trail running, and if I had more access to trails, I'd get out there more.
Why Train on Trails
Strength: Running hills will build your strength tremendously. That extra strength will translate to speed when you then run flats.