Well, that was certainly the propaganda by slave owners at the time. However, there are at least two lines of argument that make it pretty likely that "nice" slave owners were few and far between.
The first is that in the thirties anthropologists interviewed a large number of former slaves, using both a white interviewer and then a black one. While the former slaves tended to make the best of it with the white interviewer, and there's no doubt that whites did indeed occasionally show kindness towards slaves ... their stories of slavery as revealed to another black painted a pretty grim picture of slavery.
Secondly, and more scientifically, starting with he Stanford Experiments and much research since ... when you put people in master/slave relationship ... the masters will pretty quickly start abusing their slaves. We see the same things in prisons all the time, unless very carefully monitored, the guards will start abusing the prisoners, it appears to simply be human nature.
Yes, there were probably some nice slave owners, but really, a truly nice slave owner would free their slaves now, wouldn't they? Some did I expect, and some left wills where their slaves were freed when the master died, but for the most part, no, masters abuse their slaves.