DownwardDog
Riding a Brompton with a power meter.
The jury is out apparently. Shimano and Campag still say to avoid:
Is cross-chaining disastrous? Find out what the manufacturers say
We ask Shimano, SRAM, Campagnolo and FSA whether running the chain at extreme angles is a crime against cyclingroad.cc
It's not just efficiency, it is (or was) added stress in components being used at an angle.
The latest firmware release for 8000/9000 Di2 locks out certain cross chain gears by default. I suspect this is to reduce warranty claims by ham fisted shifting at high torque/low rpm in those gears.