I’ve been commuting between HK (where I work)and Bangkok on a weekly basis for close to six years using a tourist on arrival visa without any major issues. I have a Thai wife and family in Bangkok where previously I had a work visa for 10 years.
However i Just returned to Suvarnabhumi from a business trip to Phnom Penh last night and Cambodia Airways in Phnom Penh initially refused to allow me to check in as I did not have a visa or onward flight out of Bangkok.
They were aware I did not need a visa but they said they had had a UK passenger returned from Suvarnabhumi earlier that week and were not allowing similar passengers to fly unless they either had an onward flight booking or US$500 in cash
After some discussion they allowed me to check in after withdrawing US$500 from the ATM.
Arrived in Suvarnabhumi and sailed straight through immigration without any questions!