I own both a RadRunner 1 (RR1) and, more recently, a new RadRunner Plus (RR+). The RR1 with the single speed does fine on long smallish hills on pedal assist 4 (PA4) -- the highest PA level for the RR1 -- but struggles up steep hills of more than about 50-100 yards (without significant pedaling assistance from the rider). There appears to be a certain gradient -- not sure what the exact figure is -- where the torque on the RR1 pretty much gives out. As rumor has it, upgrading to a Bafang "true" 750w motor (~$250 USD) may improve the torque/climbing power of the RR1 dramatically.
The RR+, by contrast, and in my experience thus far, will kill just about any hill in a lower gear at PA5 (the highest PA level for the RR+). Of course with both bikes, the steeper the hill the slower the climb rate. But as it stands, I am not afraid of any (non-sherpa/Rocky-Mountain) hill with the RR+ (and I am no Olympic athlete). My point here is that the gears on the RR+ make a significant difference -- more so than I originally expected. Also should go without saying that conditions such as rider weight, wind, temperature, etc., will impact the performance of these bikes. And just to qualify my earlier remarks, I am 6' tall and weigh about 210 lbs. As of yet, I have no experience carrying (human) passengers (though I do frequently carry lots of baggage, including towing a bike trailer filled with groceries).
Also be advised that climbing a lot of steep hills at a high PA level will burn your battery range fairly quickly -- nice having two Runners now since I can always carry a spare battery for longer rides (40+ miles) without suffering from "range anxiety."