I doubt you'll see many good players using Polyphemus. He's ridiculously slow. His special is inferior to the regular Cyclops. A hero is a specific counter unit. It needs the mobility to get to enemy MU's. So he does 90 damage a second to MU's, that's great. Hippolyta however does 72 damage a second to an MU from the moment she enters range. While Polyphemus is wading through a mess of units and getting hit by every enemy ranged unit in the area.
Which one is more useful? Which one is more affordable? Which one trains faster in a fight? Easily, Hippolyta, on all counts.
The ranged greek heroes - all second age ones - are usually the most popular among better greek players, simply because they are the most effective, and the easiest to keep alive. Obviously, against an enemy using a lot of MU's - rarely seen in my experience outside DM - they will build all the heroes at their disposal, and keep replacing them as they die. But you will most often see either either the second age hero, because it's ranged, or the first age hero, because it's produced almost instantly.
On the other end of the spectrum, Bellerophon is considered the most powerful of the Greek heroes by most better players. His jump attack forgoes his need to wade through enemy units to get to MU's, and that jump attack also does a whopping 600 hack damage to MU's. And he can use it every five seconds, providing you micro him. (He can't use his special on a unit within four meters range, there has to be a small distance between him and the enemy.)
TheShædøwDåwn
If you're like me, then it's possible you're a clone generated from my stolen DNA. I suggest you turn yourself in for destruction immediately.