I think more than the tactical change of Bazball is the mentality switch and I think that has been an unequivocal success, because somehow McCullum has managed to make every England player believe in their core that they are as good if not better than their counterpart opposition. This was just simply not apparent during Root's captaincy and was evident with defensive bowling (we aren't good enough to get you out), defensive fields (we hope you make a mistake) and defensive batting lineups (Burns opening, saving Root, Moeen at 7 etc). It was always damage limitation from the outset, and was laid out to ensure that as many mistakes could happen whilst still being in the Test match.
Bazball is about backing yourself against your opposite number. I'll get more wickets, I'll get more runs, it won't work every time but because I believe I am better it will come off >50%. And this has made all the difference with the results. You can see this more as England grew into the series vs Australia as they actually started to believe their own hype, it was almost farcical towards the end for the Aussies.
Mind games. Australia were an amazingly talented team in the naughties, but it was their mindset which made them better than Pakistan, Sri Lanka, India all of which had world-beating players.
So bazball didn't come into the Ashes without question marks.
