This seems to me a bold claim to make. Diablo III's launch was, in many ways, a huge new service for them, a new infrastructure to test and maintain, and they were quick to fix issues with it. Compare this to the launch of The Burning Crusade expansion pack -- it was a rough launch! -- but over time, they managed to smooth their systems out such that now, even a content launch as huge as Mists of Pandaria went smoothly.
WoW is a different game, but it illustrates their iterative healing process. Since the Wings of Liberty launch went smoothly, I wouldn't expect anything less of Heart of the Swarm, as it's being released on a system that's here, ready, polished, and tested. We might see a few hiccups, but nothing close to Diablo III.
I'd hardly call Battle.net an infrastructure that Blizzard is "just not competent to build or manage."