The obvious reason to back that claim would be his failure to get his team over the hump, but the bigger black mark on his resume is his inability to be flexible. All great coaches have a plan, but if their plan is failing they will make alterations.
For years Reid has held fast to his belief that it's the system, not the players that make a team. He's partly right. Great players won't produce as well in systems that play to their weaknesses rather than their strengths. For instance, if Joe Montana was asked to spearhead an offensive attack that relied on the deep out and the go route he likely wouldn't have four Super Bowl rings.