Once the REx does turn on, it likes to run at least 10-minutes so that it is about as fully warmed up as needed to ensure it can burn off any moisture that may have accumulated in the oil and purge the muffler of any condensation. It takes that long before it will also allow full throttle, if it is actually needed to help hold or recharge the batteries.
Some of the early issues with the REx were related to moisture accumulation in various bits. Possibly from lack of use which then prevented it from actually starting up if needed. Whether that would be an issue in some places (dry desert of say AZ) or not, can't say. It would seem maybe to be more an issue for people living near the coast, especially where it is hot and humid.
2011 535i x-drive GT, 2014 i3 BEV