Big fan of the yepp mini as well. So nice to be able to converse and point things out to each other. Really sad my daughter has just out grown ours...
+1 for the Yepp Mini up front. Both kids loved it and would have great cruises around. I tried a back mount once on a rental and had to deal with the constant whinges from the little fella up back complaining he couldn't see (fair enough I say).
Crashes, well I never went full-on mtb with it, but the bike did take a tumble once or twice. Wasn't really an issue as he was strapped in with a helmet and the handle bars always hit first, as it was mounted to them.
Mounting was a (small) challenge. I couldn't have the bar from seat tube to head tube as there headtube had no space to mount. I went for the stem mount, but the u-bolt onto the stem didn't work as I had a 70mm stem with a square profile. I ended up bending up my own bar to the right diameter & adding some thread to the bottom to make my own u-bolt.
The stem mount does restrict steering a little bit, as the leg extensions for the kid run into the steerer. No issue if you're competent on the bike & expecting it, but something my wife wasn't comfortable with.
Another issue for my wife was reaching around the kids. You need broader shoulders and longer arms to make that work comfortably. Not an issue for me & never felt they were in the way though.
In combo with a balance bike, you can't go wrong. We'd right out there with the little guy under his own steam, then when he gets tired, put him in the seat with the bike on the back & head home. Many great times. He was obviously quite comfortable & relaxed in it too & loved the wind in his face.
I had used a trailer set up with the first, but had all sorts of problems with it being wider than the bike, not being comfortable for him & kicking up dirt off my wheel. IMO, the front see is much better overall & far more versatile.