A lot of this article is quoting Musk about how difficult the Falcon Heavy was, much more difficult than what they expected.
There's certainly no reason to doubt him when he says it was harder than expected, but I do find it interesting that the Delta IV Heavy [1] has an essentially identical configuration and has been operational since 2004. Maybe that rocket had similar difficulties, or maybe the large number of engines on Falcon Heavy make a real difference (each Delta IV has one large engine rather than 9 small ones).
I guess I'm surprised that Spacex was surprised about the difficulty.