Any gun is capable of being carried effectively for self defense, if you understand the capabilities of both yourself and the round being fired. Yes, some are better than others, and some are overkill. Just know the point of aim vs. point of impact, understand the penetration and expansion of the round fired, where to hit to achieve maximum damage, and practice at different ranges to verify you can hit the required poi under different situations.
Back in my 20s I went to a range with two of my uncles. One was retired Marine and the other a Army Reservist/Police officer. They carried larger calibers and I had my little pocket .22 cal. They were effective at longer ranges than me so long as they had opportunity to take aim. But when I talked them into running drills where they needed to fire quickly, I could hit the longer range targets that they missed. When you take away their time to take aim, their effective range was less than mine with my little pea shooter.
Under those circumstances, so long as I understand the round and the required shot placement for maximum effectiveness, my little gun was as effective as theirs, if not more.
A cannon is ineffective if you can't hit the target.