This is kinda Edgerunner's fault for how they depict cyberpsychosis. Especially with the injections that stabilize David. It's not this linear, more augmentations, more strain, you go crazy sort of thing. It is a combination of a sort of phantom limb phenomenon resulting in people viewing themselves as less and less human, and thus becoming further dissociated from reality. Along with the stress it puts on your body, exacerbating pre-existing mental issues, like really anything can. Drinking too much, working out too much, etc. There are both examples of people in lore who just have their shit together enough to not experience this, and people who are already so dehumanized that the effect doesn't bother them. Smasher was already a high-functioning psychopath who didn't really care about his place in the world or how he relates to other people, so it just doesn't bother him.