I've had horses considered green that were safer than many other horses described as "well trained". In those cases, green didn't mean acting stupid, wild, or untrained. It just meant the horse knew basic cues, but needed work.
Green horses aren't for beginners, but I've come across some really great ones too. Right now I have a green mare who is incredibly solid and sane, just needs work on her cues. Sometimes I feel safer on her than I do our gelding because he knows much more than I do. Which is interesting because well trained horses can also pose a problem for people! It took me a long time to get used to a horse so sensitive to cues. Every time I ride him, I feel like I discover something new that he knows how to do. That can be just as dangerous for a beginner IMO!