Not all behavioral problems in de-clawed cats are due to the de-clawing. However, I truly believe that some are (or if not, the timing of the onset of the behavioral problems is an amazing coincidence). And if they are caused by the de-clawing, they're almost unfixable. Maybe meds will help, but nothing else, really. That's the big problem. Any other cause would be fixable.
One of my cats is de-clawed (came that way). I have no idea if it was done "properly", as I didn't know him at the time. However, his GAIT (spell it right

) is severely affected (he walks funny, all back on his "heel"), and his litterbox habits are quite bad (no health issues that we can find). It feels like he has no bones in his paws at all. The last de-clawed cat (same situation--older male, dumped at my house by unknown persons) I had was the same way, and the older de-clawed female my mom fostered really did have no bones in her paws (confirmed; there were complications requiring extra surgeries, eventually removal of every bone in the foot), and, although she had fine litterbox habits, she sure hated kids and had no tolerance for handling, and would bite quite easily.
Sure, I know a few de-clawed cats with no obvious issues (they all walk funny if you know what to look for, though), but statistically they do seem much more likely to act out when stressed.
But the main reason I object to de-clawing is that the cat is rendered helpless forever. I have known several de-clawed cats who were killed by dogs because they could not climb a tree fast enough to get away. I do not know any indoor-only de-clawed cats (except mine), and have never known a cat who was indoor-only its entire life. It is unrealistic to think a cat will never have to defend itself in 20 years of life.
I also knew an older male who was deeply distressed by the loss of his claws. It was obvious by his behavior (he would try to extend his claws, when nothing happened he would start howling and attack the nearest person). He became very aggressive and depressed. He tried to stop eating, but they force-fed him. As soon as they let him out again, he ran in front of a car (keep in mind he had been a stray in that neighborhood for years and knew how to handle the traffic). I truly believe he committed suicide. So I do believe that at least some cats experience mental distress from being de-clawed. Kittens may not know better, but adult cats sure do.
And unfortunately, it's impossible to know how a cat will respond until it's too late. And this frequently means the cat's life.