If you follow their guidelines and don't try to manipulate the system then they won't cancel your ads. I have never had one canceled before. If you did, then you must be doing something like redirecting to different content than what is in your ad or something along that nature.
I thinks if you are violating TOS then it's may happen or otherwise you don't have enough funds for Advertising campaigns.
Yes, our ads get stopped all the time due to "editorial". It's not a big deal since we normally load several ads at once. Yahoo doesn't like things like exclamation points in ad text, or special characters like bullets. They also have different rules than Google about trademarked terms.
I quit using Yahoo years ago because of horrible interface changes and intrusive annoying ads. I have switched to Google and would never go back. I really don't see how this deal helps Microsoft, I would use Bing over Yahoo any day.