Thai taxis are notorious for off meter fares, but a savvy tourist can avoid them mostly (flag down a cab, don't take one waiting for you or worse, offering a ride).

I've made it work for me when the legit cabs had a long wait - I offered an off meter fare and I got instant service.

I once calculated the fare using a taxi fare site and the verbal off meter rate was lower than the estimated one. I gave him the higher rate.

Off meter isn't so black or white, that said I always prefer uber or similar regional cabs.