Using feet is so much better than arms
The idea is a vast improvement over trying to crank 90 RPM with a hand held version. My arms would get so tired it was nearly impossible to keep going. The pedal version is definitely better because of the obviously stronger leg power. The one thing that I would say that would make this work so much better is the addition of a gear, so that you don't have to physically spin your pedals 90 rpm -- a gear that allows you to spin anything less, such as 45 or 60 rpm, while the gear transfers the speed to the charger so it turns 90 rpm would be a lot easier.
The reality is - spinning 90 rpm with your feet on the pedals takes a lot of energy and concentration, it's practically the equivalent of sprinting, and you can't do it "mindlessly", as soon as you think about anything else and try to let your body go on auto pilot, your legs slow down to just a little less than 90 rpm, and I presume that it just doesn't charge much or at all at that pace (I noticed my charging light wavering when I slowed a bit).
It's just hard to keep up a non-stop sprint pace for 10-15 minutes or more, and the actual resistance isn't that hard (way better than with your arms), it's just the speed is tough to keep up.
I assume that an extra gear to translate the speed would add a bit of size, weight, and cost, but I would pay it. It would make the whole thing much more effective.
Proseasier to spin with legs and arms
Conshard to keep up 90 rpm for as long as it takes to charge a device or battery
Best Usesblackouts and power outages, possibly camping
the amount of charge you can accumulate realistically would not be much more than your phone, flashlight, maybe a radio etc. but you'd end up being very judicious about using those devices because of the difficulty in keeping up the 90 rpm for the 10-20 minutes or more to charge those devices.