I usually see no correlation between epic and tempo/technicality. To me music is epic when I picture the songs music video as, well, epic. One thing that springs to mind is the scene in LotR: Return of the King where the riders of Rohan enter the fields of Minas Tirith as the sun sets down, bathing the fields in golden light. That's my understanding of epic =D.
The word epic can be translated to something like majestic, or grandiose, and that's exactly what I feel when I listen to Human Fortress. There aren't many bands that I've come across that can create this kind of smooth, simple sense of importance and majesty, and I love them for it.
There are of course different types of music that could be considered as epic: like Gamma Ray, Nightwish, Kamelot, Lost Horizon, the list goes on and on.
I do agree that their music isn't very versatile, but I also think that's one of their greatest strengths. Defenders of the Crown still stays at the top of my list ^_^.