It's not debatable.
During his tenure: Did we win a division or not? Did we make the playoffs 3 times (almost a 4th if it weren't for a blown call)? Did we win the 2nd most games in the NL when he was at the helm?
What are the criteria for a 'championship' team? Does winning not count?
There's no disputing that Wren built some very, very solid teams with a limited payroll while producing a lot of useful talent.
Can you make the argument that Wren was leading the Braves down the road to perdition? Sure, a lot of people definitely seem to believe that.
But to say that it is 'debatable' whether or not his teams were championship caliber? Nah.