The biggest downside in Thailand is the way westerners have been treated by the Thai governments. I have been here now for nearly 20 years and comparing Thailand 20 years ago to now, things are much worse today.
If I knew that I would have to do things now that I did not have to do 20 years ago I would never have settled down here. ie being treated like a common criminal with this 90 day reporting nonsense, but people who arrived here around 20 years ago will know what I mean. I always wonder how it will be for westerners here in 20 years time.
In saying that, the good still outweighs the bad compared to the UK.