"but the first flight of the first A350 was just as uneventful. And the A330."
Several things have helped, many of them IT related.
AFAIK all bit aircraft will be done on CAD systems and they can do automatic clearance and fit checking, Part of the original purpose of rollouts was to find what had not been done right and "shim" it together/apart as needed.
The mfg by "hogging out" large lumps of alloy or laying up large pieces of composite eliminates a shedload of fasteners and their accumulation of tolerance IE all the little bits being slightly out adding up to a whole part that's a lot out.
CFD (certainly below M1) is pretty good so any suspect features that may make handling difficult can be tested and either redesigned or flight rules adjusted so they are not a hazard (at least in early testing).
This results in generally much less drama with new types.
However you can get new problems as well. Airbus had issues when their 2 design centres had different versions of the same CAD SW. IIRC the datum points (where exactly is "0" in the X, Y, Z dimensions relative to each drawing) was just a bit out. This caused months of delay.
Possibly the ultimate case of "More haste, less speed." (-:
Biting the hand that feeds IT
