i.mx8 Nano supposedly can idle at <0.2W, with max power somewhere around 2W. They have also announced ULP version if i.mx but that seems somewhat vaporware at this point.
As for the relative unpopularity of Cortex A3x series, I suspect that might partly because Cortex M series has been creeping into similar performance territory, like for example the i.mx rt mcu on Teensy 4.x, which has Cortex M7 running at 600 MHz.
As for the relative unpopularity of Cortex A3x series, I suspect that might partly because Cortex M series has been creeping into similar performance territory, like for example the i.mx rt mcu on Teensy 4.x, which has Cortex M7 running at 600 MHz.