Short answer: Sure they are!
Long answer: Owing to a vertical mouse’s ergonomic build, users of vertical mice are less likely to develop carpal tunnel. In cases of preexisting carpal tunnel, they're less likely to experience agitation of the condition. That said, established overuse of desktop-geared mice (and keyboards), regardless of how “ergonomically friendly” they are, can lead to carpal tunnel syndrome.
As previously stated, the ergonomics of a vertical mouse can actively prevent pronation. Pronation of the wrist/hand can lead to tightening of the joints, which increases pressure on the entire lower arm and can result in painful carpal tunnel syndrome.
For a more in-depth and medically qualified explanation of carpal tunnel symptoms, causes, and mitigatory methods, you can fall into the syndrome’s WebMD rabbit hole here.