Been around horses in one way and another all my life and I do have reservations about racing. And indeed some other equestrian sports. Racing life is about as far away from a natural life for a horse as possible, because they need to be in tip top condition and uninjured. Horses are ridiculous at injuring themselves in the most mundane of situations, so highly strung racing tbs are even worse! They're usually kept in a lot and not allowed turn out in a herd environment, and are fed diets which help them perform to the best of their ability but I don't think do their incredibly sensitive gut any good at all. They usually are kept to the highest stable standards though, expensive bedding and rugs to keep them warm, massage, heat lamps etc - and yes, welfare standards are high for racing and other professional equestrian sports - a poorly looked after horse will not perform well.
They are usually destroyed on breaking a limb, but this is true of most horses breaking limbs, and it happens as I've said in the most mundane of circumstances, I've known several get into Argy bargy at the gate all wanting to be in first and get kicked, panic and get injuries, start playing on a spring day with each other and fall or get kicked, all these horses were PTS as it was the kindest thing to do - horses suffer a lot being kept still for months on end so a limb can heal, they often stop eating and drinking, pain relief can be an issue, I have known 2 that could have restricted movement that recovered and lived as pets after breaks, some owners and vets tried to only have the horse destroyed a week or so down the line, others did it there and then. It's normally the vet that ultimately makes the decision as such, they know what the likely outcome will be from the type of injury.
I think many horses are broken in too early, but racehorses definitely so. And there is a difference between starting and breaking a horse. The ones I've helped with have been started, not broken. I don't like that term at all.
My other concern is how retired racehorses are given away, people don't realise the specialist care any horse needs, and times that by a hundred for a racehorse. I've nigh on 40 years experience and I wouldn't try and retrain one. But they're taken on by people who've had half a dozen lessons and think they know what they're doing - disasterous. The trainer's and owners need to be more accountable.
There are many other acts of cruelty in the horse world that sadly don't get as much publicity, Google Rollkur in dressage. Truly horrible.
Another thing is horses being tethered and fly grazed, another horrible practice. These horses aren't even getting the basics in life.
There's also an equine obesity problem, over feeding, over rugging, keeping horses in because the owner doesn't want them to get dirty.
The horse world has changed massively in the last 40 years, some wonderful advancements that help, some awful practices that harm.