They're certainly tough enough to ride off road with. However the geometry of the 26" one at least is quite steep (72 degree head angle) so may be worth checking. I have an old 26" wheeled frame, as does my GF, that has been a long distance rough track tourer (totalled about 600miles in that guise), MTB with 100mm forks and front disc, single speed commuter and various other things since I bought it in 2004. It's a bit battered looking, but still completely sound.
I have found that they do ride quite heavily even with an ok build and aren't as nice as most of the other steel frames on the market. I'd say for general messing around on and commuting - great. For more serious MTB - look elsewhere.