For low-power stuff, with small footprint, ST is at least one generation behind people like Silicon Labs EFM32, Nordic Semi and Dialog. Nordic and Dialog even have built-in radios... And, they're still cheaper than the low-power variants of STM32s..
Not sure what you mean by one generation behind? As far as I can see the STM32L4 devices have similar, if not lower core power consumption to any of the Silicon Devices Geckos including the latest Jade and Pearl products which are much slower, have only 32K RAM and have less comprehensive peripherals apart from the crypto unit. The L4's ADC is much faster and uses rather less power (at the same speed), but I've not looked at the power consumption of other peripherals.
Neither Nordic or Dialog publish power consumption for their latest devices but Nordic are claiming 58 coremarks/mA for the nRF52 which is impressive - twice as good as the L4 (though reduced from the 90 they originally claimed). However mAs aren't mWs - they could be quoting 3.3V figures rather than 1.8V. Dialog use the M0 core so probably nowhere near as good as the L4 for mW/MHz.
The L4 parts are expensive but I assume that is because they are higher end parts and targetting a different market with USB and LCD peripherals in some parts. I would guess that the range will be extended to cheaper, lower end parts eventually but I've no idea if they will get cheap enough for the wireless sensor market.
As for the packaging - the smallest currently offered is WLCSP72, 4.4 x 3.75mm compared to the smallest pearl/jade devices which are 5mmx5mm.
ST don't yet offer integrated radio but they have licensed Semtech's LoRa sub GHz radio technology but I've no idea when they will come to market. Another thing the L4 lack are built in DC-DC convertors, unlike the others, which saves board space for battery powered devices.
For low power I doubt that Ambiq Micro can be beaten - they are claiming a breakthrough 35uA/MHz for their sub-threshold technology which is way better (3 to 4X) than any other Cortex product that I've heard of. Definately one to watch and probably worrying ST and Silicon Labs.