From what we know now, I don't think the e-Power is any more (or less) than a series hybrid ICEV.
The video below shows battery discharge and both ICEV and regen charging, on the far right side of the dash:
It obviously maintains the battery charge in a narrow band, near ~80% (?) while you drive.
At ~21 minutes into this video, you can see the RPM efficiency range:
So, the ICE apparently will run a higher % of total drive time than shown in the first video, when under higher sustained kW loads, but at the same ~optimum efficiency, at ~2,500 rpm.
I doubt it will have any problems climbing mountains, probably by drawing down battery capacity to lower levels than we saw in the first video, and maybe also by running the ICE at higher RPMs/lower efficiency than optimal on very long high-speed grades.
I would expect this serial design to exceed parallel hybrids which run their ICEs at less-efficient RPMs (and lower thermal efficiency) in city
use, which is why it scores such high MPG in the slow speed/stop-and-go Japanese test cycle.
Whether e-Power will be able to ~ match parallel hybrids' MPG at American freeway speeds, will be interesting to see.
Major questions remaining:
What total/available battery capacity?
Is the fraction of available battery accessible under the driver's control?
Just how quick is it?
If Nissan can sell an e-power that accelerates ~ as fast as a Bolt or Tesla 3, at about half the list price, it might be very tempting to those who want to buy BEVs primarily for their from-the-stoplight