I'm fine with yelling, especially if they are trying to make a point clear (foul language is fair game too..).
I like to know *why*. Yelling at me to do this, do that, blah, blah, blah doesn't tell me why I need to do that and how it affects my horse. I really like my current trainer because she tells me what to do in the midst of the moment but explains why later on. So she'll yell at me to shorten my outside rein and then later tell me that my horse was falling out because I had zero contact with the outside rein and it needed to be shorter.