It seems to be something of a trend (or at least something come from a couple of people) to claim that Bowser is bottom tier material. Something about that doesn't quite sit right with me, and I don't mean in an "I main the character and you just don't see his true potential!!!" sort of way. It's more that I can't see his combination of tools ever being that bad, other than, yes, you risk getting punished if you mess up because some of your attacks leave you wide open. I'm not a Bowser player so I can't speak from that angle, but he seems a little too...good to be bad?
I mentioned this some weeks ago, but I was watching a Sheik vs. Bowser matchup where Sheik had total control of the set only to end up losing because of three good reads by Bowser. Just having that in mind makes me think that it's hard to ever truly count out Bowser even if he isn't top 15, let alone top 5.
To go off on a bit of tangent from this, I feel like there's a constant contention as to the value of mechanical skill vs. reading and strategy among people, with safety often being prized by a lot of players. It makes sense because, generally speaking, the safer your character is the less chances they have of getting hurt bad, but I think that Smash 4 has tried to make as many strides towards read-based styles, which is why you have such disagreement over a character like Ganondorf.