"The cruelty aspects"....what cruelty aspects? Please be specific.
The huge majority of trainers and owners have the welfare of the dogs as first priority. It is in the interests of trainers to have their animals in good condition and be in a position to perform to the best of their ability.
If the trainers were not doing their job, owners would soon move their dogs elsewhere.

Regarding scarring on dogs, these are most often caused by dogs fighting, or perhaps running loose into wire fencing etc.
Scarring rarely occurs on the track, where surfaces are always well protected. The most common injuries are hock and wrist bones due to the dogs taking the bends at speeds approaching 40mph.
Most tracks are designed to alleviate this problem, by making the bends more gradual and increasing the banking. The new track at Toweceter is a good example.
A dogs temperament can be affected by any number of things which have occurred during its life, to blame the temperament of a withdrawn dog on the cruelty of a trainer is ridiculous...unless you have proof that cruelty occurred.