how easy is to get such letters?
well you obviously must do something (nearly anything) to get a warning letter, but it's not always deserving, its completely open to personal opinion wether you get one or not. and if your a FO it affects allot with your career in the airline. Even if you have completed the upgrade interview and jump thru all the training hoops… then get a warning letter and it's all over. thats just the way it is. learn to live with it, learn to deal with it, or don't come here because it's not going to change anytime soon.