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Well, they had gears, suspension and 55mm+ tires. And disc brakes.
Also it was my first time really offroad.I could hold up with them just fine (read going as fast as I could and sometimes pushing uphill) until we came to a strip of singletrack with lots of roots. I got shaken up pretty badly, had to go slow, push lots of times.
At the end, there was a downhill section with pretty brutal gravel. Going down there with 40+ km/h I hit a rock with the rear wheel, kicking it sideways. I somehow stayed on the bike and did not die, but got a pinch flat and a wheel out of true.
I did the same route on a proper mountainbike lately (1x10 rigid steel 29er. But at least fat tires), and it went way smoother.
csb.
Went mountainbiking with friends once on my SS Cross check with 42mm tires. Would not recommend.