Out of all the odd claims in cricket in the modern era I find this the most puzzling.
On paper no one would question him due to an unmatched record, but surely in our generation it is unwarranted to make such claims as none of us witnessed him or saw what he was capable of. For this reason I refuse to call him the greatest and will only refer to Don as the greatest of his era, but there are more questions beckoning behind this claim.
In Don's day there were no third umpire runouts and we don't know the size of ground he played on surely he would have at leat one runout going against him. We also know that batsmen of our generation play cricket under far more stress with more globe trotting for longer periods, shorter intervals, and more stress with cricket being a career for some and extra pressure from fans, etc. More importantly the variety of opposition and wickets is a major factor that surely needs to be taken into consideration and lastly even umpiring is likely more professional today than yesteryear.
I feel comfortable with calling Tendulkar the greatest ever and know zero about Bradman beyond wha'ts on paper.
Final factor (a big one) hours of video footage today that teams look through to expose potential weakness areas that batsman have. Look at Phil Hughes as an example.