Sure, it wasn't pretty all the time, but Rockets are a team that can be tough to guard, and despite Harden getting to the line a lot, his 6-16 shooting indicates he was covered pretty well.
Also of notice, the Spurs held the Rockets to 40% shooting from the field (6% lower than their average) and 34.5% from downtown (2.5% lower than their average).
The rebounding was perhaps the area where the Spurs needed to do a better job (Rockets +6), but credit Asik: he was physical and dominant in the rebounding department.
All in all, the Spurs will win more games than lose playing this kind of D, IMO. The 18 turnovers is probably where this game was decided, but that's an aspect of the offense and warrants it's own analysis.