Why the 1929 battlecruiser designs convinced neither way

Why were the 1929 battlecruiser designs unconvincing?

They tried to sit in the middle and could not commit successfully to either role.

The 1929 battlecruiser concepts attempted to occupy the center of the spectrum instead of committing to battle-fleet or cruiser work. That compromise left them unable to excel in either direction. They sacrificed the decisive strengths found at each end of the spectrum. A design intended for both jobs can therefore become poorly suited to either one.