One thing I haven't done but have meant to is to try it again in winter conditions. To see if the greater temperature differential at 0F would help (I tried it on a 60F-ish summer evening). My SWAG is that it would help 20% or so for electrical output, but I don't know if the heat transfer is more limited on the hot or cold side. The cold side benefits from the fan blowing air past it. The hot side is a pretty small surface area and while copper, seems mostly to be radiant heated. Fins on the hot side would have helped.
And my SWAG is just that - a scientific wild-a$$ guess (a WAG with numbers attached). An actual field test is perhaps called for, but I don't know if we'll get anymore 0F temps this year.
On the flip side, I haven't tried in African / remote South American conditions. At an ambient temp of 100-110F, it would not do as well, but I haven't quantified that, either.
Perhaps instead of a USB charger, it should be viewed as a self-powered, fan-stoked stove. As such, 50% more diameter and height would have given a burn chamber with 3.4 times more volume, not needing such frequent refueling and not needing to split/cut fuel so small.
So I wish they'd done either a BPing version - lighter materials, or a base-camp / village-sized one. The electrical component is the only part with any technology in it, and it could be mounted on 3 or 4 sides of the fire chamber to increase electrical output without increasing the need for fuel. That would make a better base-camp or village version in that 4 devices could be charged at once.