I too have rehomed a dogs, either because they weren't working in my particular environment (or the career change guide dog I trained as a service dog). Basically, it's how the dog is raised that will affect how successful a rehoming will be.
If you shelter the dog, and it's never exposed to anyone but you and your family/home, is with the same people all the time, then yes, it could be difficult for the dog to adjust to a new home. But with show/trialing dogs, (reiterating a lot of what has already been said) they are used to going and staying strange places, with strange people handling them, and they adjust much better to the new situation than one who hasn't experienced these things.
That's not to say I wasn't attached to the dogs I've rehomed, but I interviewed the prospective owner, had them meet the dog and watched the reaction. With each case, the dog seemed to understand what was going (yes I'm anthropomorphizing, but I can see it in them) and and they happily went with their new owner/family. I made the decision in the dog's best interest - if they didn't want to do performance work or conformation and that's where all of my weekends are spent (with my other dogs who DO enjoy those things), that dog isn't very happy, now is it?
Now, I do have issues with kennels who breed just to rack up champions and automatically place them after they are finished. I know a few like that and that's a whole 'nother story. The dogs adjust, but the reason is wrong. They are just producing the dogs for the single purpose of increasing the number of champions their kennel has produced. However, this is not the norm.