DUIs aren't the show stopper they once were. depends on how long it's been and what the outcome was (deferred adjudication, diversion, wet and reckless, etc). The main issue is that you become a liability flying in to Canada (or your lack of ability to do so).

It seems substance arrests are a different issue. Almost every TSA and FBI background check red flags those, therefore the airlines are abiding by someone else's rules, rather than their own.