Disadvantages: You may not work - not eligible to apply for work permit if holding Class O retirement extension. If extending based on bank balance, requires minimum balance of 800,000 baht for 60 days (initial) or 90 days (renewal). If extending based on pension, requires certification of pension by your country's Embassy in Thailand.
not totally correct, here in Huahin immigration require a cert from the bank that you have the funds in your account on the day you apply for your visa, nothing said about 60 days....
just my experience last month.
I went for the retirement visa as way less paperwork hassle.