Some stories I read said that BMW really didn't want to include the REx option at all but it was pushed by marketing. The vehicle is designed as a city car for commuting, not as a cross-country one but it's a tribute to the design that people can use it that way.
The thing is a serial hybrid meaning that when the battery is too low in charge, extended needs of high current just cannot be sustained. The motor is a 170Hp one, which is about 126 Kw/hr. The engine is only about 34 Hp...enough to help maintain the battery level, but by no means enough to power the motor to its fullest extent. It can handle a passing maneuver, but not climbing a hill momentarily augmented by the battery.
If anything, BMW was negligent in fully explaining what and how the thing worked, but then, how many people would have sat around to listen? There's been many decades of expectations about how cars work...the new ones do not work that way. Maybe it's the engineer in me...I don't see this. The vehicle will be warning you that the charge is getting low. Ignore it at your peril.