Top 10 RAAM finisher, State Road Champion, STRAVA yearly elevation climbed No. 1 and Cat. 1 raced, Brian Toone, has ridden unpaved fire roads, single track and cow paths on his TREK Madone with carbon fiber 50 MM wheels.
Just sayin'...
They old saying, "If it does 'everything', it does nothing well.", comes to mind though.
mpre53, one of the kids I train with used his 26" hardtail on a 50-mile road ride we did. I don't know if his front end locked out or not. IIRC we averaged around 18 MPH and he was working hard, but had no trouble keeping up with the road bikes. It can be done if you have the legs.