Rejections at Mae Sai - and as Todd notes they simply won't let you exit vs stranding you in Myanmar, are rare. Presuming you haven't done your two land border bounces in the year you should be fine.
There are more reports recently, however, of people having a hard time getting 30 day extensions at CM Immigration if they have a long term history in Thailand on visa or visa exempt entries with extensions.
Joseph Michael Pedro III and for the record - now here on Non Imm B (BOI) and work permit. I just want to know whether overstays are actually NOW considered in Penang applications.
if economics are your consideration rent a car - easy two day drive to the border (can be done in one if pushing it). 5-6 days car rental is going to be about 6k give or take vs ~12k for you flights. With kids route 12 through Petchabun is elephant country and theres some nicd places to stop off.