That may well be true, but not from what I've studied about either. (though there is certainly more to know than what I know) Certainly the Mongols moved quickly by horseback but did not always fight on horseback. The arguement of lack of forraging has been brought up before, but given the terrain of Mongolia in the first place one would wonder how they managed to move a force on horseback at all. But they did. The Mongols, very much unlike the Romans, acknowleged and even embraced their enemies weaponry. If it was superior, they incorporated it. They even had armor piercing arrows. The Mongols were constanly outnumbered. The Romans were seldom outnumbered.
The Roman formations that I'm familiar with were all based on open field tactics to allow their large force to move, outmaneuver and dominate their opponents. Though I know of losses that they suffered in the forests, i.e.(Battle of the Teuoburg Forest A.D. 9 in Germania)(Battle of Adrianople, A.D. 378 under Emporer Valens against the Goths), I don't know of any major victories. As for the mountains, Sparticus gave the Romans fits by traveling through the mountains, If he had followed his original plan of going north through the Alps, they never would have caught him. Their attempts to that point were one defeat after another. They trapped his army in the Petelia mountains with two legions only to be completely wiped out. It wasn't until he turned back south and got sandwiched between two armies at Brundisium that he was finally beaten.
I'm fasinated with history but am by no means an authority. I love to learn. Your input is appreciated.