“In my personal opinion” sounds a lot like “don’t take this so seriously that you judge me when I say this, because I’m human and make mistakes too...” which does not sound so bad, if not for its unprofessionalism.
If a Doctor said, “in my personal opinion,” he should be punched in the throat and told, “These people depend on you with their lives, God dammit! People need medical advice, not medical maybe.”
The same goes for Design professionals. Clients depend on us.
Humble we are in the eye of creativity & its infinite possibility, but confident we must become in the eyes of people who trust us with their ideas.