We haven't seen that many of the Age of Legends channelers use their full training because they are often in small-scale battles or trying to infiltrate while unnoticed, but Lanfears' attack on the Aiel was the one time we did see a Forsaken in full-scale battle mode. We know she is immensely strong, but her appearance in that scene was very similar to what we saw in Rand - we don't know how many Aiel she destroyed personally, but it seemed to be in the hundreds or thousands and she literally had to walk through their camp to reach her destination, meaning she did all of this in a minute or two. Semirhage also wiped out the entire Imperial family and their attendants, and she may very well have had to destroy hundreds of people in a closely guarded fortress.

I think the Age of Legends channelers had techniques that allowed them to use their full potential without exhausting themselves, and Rands' connection to the Creator made him seem more inspirational.