The REx can maintain city speeds and most legal highway speeds in the USA. It's just that most traffic exceeds the posted speed limits. You'll run into issues if your SOC is low and you then try to maintain high speeds up a long grade, or exceed the capacity of the REx to maintain (generally, a bit over 70mph). You may never have driven a vehicle with a 34Hp motor, but that's what you've got once your battery is low...if you're old enough, one of the original Bug's was in that range. Okay on the flat, but you definitely couldn't storm up a grade with one. At least on the REx, until the battery is on its last legs, you can draw from the battery's buffer to make up for the deficit. If you can get to the top, you can regain some of that energy on the way back down off of a long grade. High, extended speeds are like constantly climbing a grade...your battery buffer will gradually continue to be depleted. When I got into mine today...it said 69-miles to empty. I drove 14-miles, and when I got back, it said 67-miles to empty...how you drive, where you drive, and how fast you drive makes a big difference. This trip was mostly at 35-40mph, my previous trip was partly on the highway at 70mph...gives one pause to think about the impact of speed.
BMW could have made the REx engine larger, but for a car designed for use around the city, it's perfect (IMHO). When you try to use it for long-distance touring, it falls short, but it can still do it just like using pliers when a wrench would work better. Almost no other hybrid out there is a serial hybrid like the i3. Even the i8 is a parallel hybrid, as are all of the other, current BMW's that have batteries. The new x3 is reported to be a BEV, once it eventually shows up. With the slightly larger chassis, IMHO, it would be better with a parallel hybrid, but the infrastructure for DC fast charging may be expanding by the time it shows up.
As to a portable generator...I'm not sure how noisy electrically, the i3's power circuits can handle. NOt all generators put out a clean sinewave. Then, you'd have to have an EVSE rated for the available power, or your charging would be diminished. I'm not all that comfortable carrying around gasoline inside of a vehicle, either. I think you'd find that a 3.5K generator could provide about a max of 10-12 mph into the battery. Keep in mind that those delivered in the USA max out at 7.4Kw/hr and can take 3-4+ hours to fully recharge, depending on the battery...it would take at least double that with a 3.5Kw generator, IF you could load it at its maximum without issues, and the output waveform was acceptable to the vehicle. Many generators are rated at a peak, but also have a different value for constant loading that is lower.