Silicon Labs isn't really motivated to get our competitor's software (ST's SimpleMAC) running on our chips (SiLabs/Ember EM35x), but since you asked the question, here's a quick contrasting of the STM32W108 vs. EM357:
* Both were co-designed by Ember Corp and ST Micro prior to Ember's acquisition by Silicon Labs and are based on an EM300 series platform using the ARM Cortex-M3 core.
* Both are footprint-compatible QFN48 packages with identical peripheral sets (and ARM Cortex-M3 cores) and 24 GPIOs.
* Both will run EmberZNet versions 4.0.2 - 4.3.0. However, after version 4.3.0, the partnership agreement between Ember and ST was concluded, and no further software stack support was (or will be) provided for STM32W108.
* STM32W108 has 128KB flash, 8KB RAM, while EM351 has 128KB flash, 12KB RAM. EM357 has 192KB flash, 12KB RAM.
* EmberZNet firmware built for any one of the above 3 ICs won't run on any of the other ones. I have no idea about ST's SimpleMAC (since they wrote that, not Ember), but presumably it would not run as-is on the EM35x variants since the EM351/7 ICs are a later revision of IC design than the STM32W108 silicon, so the registers are slightly different.