I would never fly my dog in the cargo hold - as a flat faced breed, he would must likely die. Some airlines have actually stopped allowing dogs like pugs, english bulldogs, french bulldogs and boston terriers to be flown in cargo because of the high incidence of complications due to the extreme temperature changes and air pressure issues.
A few years ago, several top Samoyeds were killed on a flight to their national breed show. The pilot "forgot" that there were live animals in the cargo hold, and didn't bother pressurizing/heating the area.
I have heard this both ways. That is, that the deaths were not really caused as said. I have also heard that the pressursation in the baggage area of planes set up for passengers is connected to the passenger compartment is such a way that there can be not differences in pressure. The poit was made with this that otherwise there would be too high of potential for aerosol cans to explode, and that absolutely any of the pump type dispensers would fill your luggage with liquid when the pressure dropped.
I do know this: When we came back from Taiwan our dog rode across the ocean as baggage. We had to take her out of her cage for it to go through X-ray and then put her back. She was not happy with the entire process, but she made it fine despite being a rather elderly pooch at that time. The only problem: It took her a couple of months to forgive us for putting her through it. What I have heard: Do not give your animal any form of sedative before the trip. If you do, then it is more likely to succumb or have difficulties to any distress due to pressure changes or whatever else during the trip as their body is less able to respond. And, there is pressure change as anybody with the least sensitivity in their ears can attest.