I thought playing for a small market with no real playoff success and not reaching that 500 HR mark that a lot of first baseman are held to could make it debatable. Obviously he made up for not getting 500 HRs by just being a flat out great hitter.
This is where advanced stats has made society smarter where you can look past some of that crap and objectively measure his greatness without actually watching him do it since his teams were hardly ever must watch.