As was the case in Legion. And WoD. And MoP. And Cata. And Wrath. And BC. So this is not a justification for the sluggish nature of BFA beta.
We are only 2 months out from launch. In the past, the pre-expansion patch would've been released by now. And here we are still waiting for classes to be designed, features to be implemented and game-breaking bugs to be fixed. What can explain the major delays in beta? Because it isn't the internal build argument.