Apple and many arm mobile platforms also have a mix of performance and efficiency cores so it seems to be a proven approach. I guess it comes down to implementation. Intel's efficiency cores by themselves (eg N series) apparently make nice little appliances, often better value than something like a RPi. I don't know how much they help their higher performance devices conserve energy.

I have one of Intel's old desktop class processors in a refurbished ex-office mini-desktop plugged into a power meter running a few services for the household and the idle usage isn't terrible. I don't understand why my laptop doesn't run colder and longer given the years of development between them.

There is also the race to idle strategy where you spike the performance cores to get back to idle which probably works well with a lot of office usage but not so well with something more demanding like games or benchmarks.