Honestly, I just got my e-power about 3 weeks ago and it's really not all that much better than my big ego batteries. They last about the same, and I don't notice too much difference between 3.2ish volts and 3.7v. I've never had a single problem with any of my ego batteries, but I've had terrible luck with the 14650's. One out of two that came with it still works and the one I bought from madvapes was defective. The charger is pretty terrible too, it requires a bit of fiddling to get the battery to start charging. The threads on the e-power aren't great either.
It also kinda pisses me off that with 3 weeks of use the silver coating is coming off the switch already. I've used my ego batteries as bottle openers and they still look like new after almost 4 months.
If it were me, I'd get a riva 1000 kit. Best of both worlds from stuff I've tried personally. I'd rather give up the interchangeability for something that requires less screwing around.
Have you tried the Kgo? They're better than the Riva 510's and Riva UNC I had.