I often hear people mention that flyball is not a safe sport. Generally, these are people who are involved in other dog sports such as agility, disc, obedience, or freestyle. They are experienced dog people. They simply haven't been exposed to flyball in a proper way.
Years ago, flyball was a sport that risked some pretty significant injury, especially from repetitive stress. Before the modern flyball box was created, dogs were trained to run fast down the lane, slam into a pedal with one or both front feet, coming to a complete stop, catch the ball as it launched, then pivot and run back to their handler. Both the abrupt stop and the hard pivot were likely to (and did) cause injuries, both acute and, more often, chronic. Arthritis in the front shoulders was common in older flyball dogs.
Flyball, however, has come a long way since then, in both equipment and training. Flyball boxes now have holes positioned to either side, instead of in the center. The front pedal is a padded surface large enough for all four feet to land on, instead of just one or two feet. Thrusters launch the ball more efficiently and predictably. We now spend months training dogs to use all four feet to create a smooth "swimmers turn" on the box, eliminating the abrupt stop and slamming of the front feet, legs, and shoulders and also the stress on the back and rear from the following pivot. The swimmers turn allows dogs to safely retrieve the ball from the box more quickly and with less stress on their bodies.
Those who have followed my blog for a while will know that I have been spending, quite literally, years training Koira to have a proper box turn. This isn't out of some desire to have a super fast dog, or win races, or set records. I want her to have a good turn so that she is able to play flyball safely and with the minimum amount of stress possible on her body. Some dogs who run slower and self-regulate how hard they hit the box may be able to run flyball for years with a poor turn without any injuries. Koira, in my opinion, runs too hard and too fast, and has too little self preservation instinct, to be allowed to do any sub-standard turns without a risk of injury. (As a note, normally it should not take this long to train a safe turn. Unfortunately, I trained Koira with a poor turn before I knew any better, and have since spent years trying to correct it.)
I would compare training a proper box turn in flyball to training good contacts in agility. It can be hard. Some dogs take to it super fast, others take forever to learn it, and some need constant reminders about how to do it right. If you don't bother to train contacts, your dog may never hurt themselves, but the risk is much higher that they will launch themselves off of a piece of equipment early and become injured. There are many different strategies for training contacts. Some work better for some dogs and less well for others. But it is important to spend the time and effort to train it properly.
All of these things are true in flyball as well, except that in flyball, the dog and the team are not penalized for a poor turn (while a missed contact in agility is penalized). This means that the handler has the burden of doing the right thing in training to keep their dog safe. They have to spend the time and effort in training, not to succeed in the sport (though dogs with good turns have much faster times) but to keep their dog safe and healthy. Sometimes, this means training a "good enough" turn that the dog will consistently perform without a high risk of injury, but without spending years to train the "perfect" turn. A "good enough" turn depends a lot on the dog. (Some dogs can perform a three-footed turn safely for years without a significant risk of injury. Other dogs may do a three footed turn in a way that increases risk of injury.)
I urge those of you who hear (or especially those of you who say) that flyball is not safe to spend some time checking out what goes into making this sport as safe as possible for the dogs who play it. Is there a risk of injury? Of course. Every time you let your dog play fetch, go swimming, or play any sport, there is a risk. The goal in all fun activities should be to minimize that risk as much as possible. I believe that is what good flyball trainers and teams do.