Using a 2-layer PCB in the application hardware is usually preferred by customers due to cost saving purposes. Still, based on the RF output power level, unwanted radiation of top or bottom layer traces (mostly VDD or digital) can occur, which causes that the application can violate the harmonic limits of the related standards. In order to minimize the possibility of unwanted trace radiations, Silicon Labs recommends to use multilayer PCBs in the following cases:
>=16dBm output power at sub-GHz frequencies
>=10dBm output power at 2.4GHz
Note that the actual recommendation depends on which standard (ETSI, FCC, ARIB, etc.) the application has to be compliant with.
One can notice that not all Silicon Labs reference design follows the above listed recommendations. EZRadioPRO reference design boards are made on 2 or 4 PCB layers based on the output power, while all EZR32 and EFR32 reference design boards are using 4 or 6 PCB layers due to the complexity of the design. In the latter case, the layout routing could not be realised on a 2-layer PCB if all digital traces were intended to be used. Of course, on a custom design where the complexity of the design is much less, 2-layer PCBs can be used for EZR32 or EFR32 applications as well considering the above listed recommendations.