I have not worked with the SITcore SOMs but have worked with the G120 and G400 in the past and almost always had vias en signals routed directly underneath them.
Nowadays, I mostly use the SC20100S in LQFP package and I never had any problem with having vias or signals underneath them.
The only thing I like to keep free from any disturbance is my main crystal. I have had bad experiences having other signals going underneath the crystal, both top and bottom layers. I usually try to keep signals away from the crystal and sometimes I go the extra mile and protect my crystal with a ground ring.
In conclusion, I don’t think you’ll find any problems routing signals underneath your SOM. There might be exceptions, like if you’re planning on using SPI or similar on super super high clock frequencies, then it might cause problems.