Actually they are more humane, especially with fish like pike and trout that have a heavy slime coat protecting them. Trout are so slippery you need to hold them much too hard for my liking. As long as we hold them correctly, vertical, or support their bellies if holding horizontal, it is the same risk for injury as holding by hand. Actually could be less risk because if a fish thrashes, fish gripper has less chance of dropping the fish. The grippers I use hold them tight enough that they don't drop, but they don't lock them down when they thrash either. I have the rapala grip on my scale, but have yet to use it. I am a bit hesitant because that one really locks them down.
Unfortunately too many pros do a disservice by how they hold bass after a catch and it is caught on coverage, many of them are guilty of this. Wish the tourney circuits would crack down on this and penalize them for it, and allow nets.
Back to the slime coat, I have definitely seen pike with infections where there slime coat was rubbed off and it looked to be to human handling. I do everything possible to protect the resource and have caught a few fish with specific markings a few times over a many year span, so I know I am doing something right.