The group set has nothing to do with frame flex, for what it's worth.
Anyways, the tubing is Unbranded Reynolds 520 that is contracted for them by the same tubing company which has the rights to make Reynolds 520. This has been confirmed by the guy who started All City.
Basically, it's conservatively butted OS tubing that should last for decades of normal use.
There may be some flex in the bb shell area due to rider and luggage weight, but nobody here will know for sure. Flex there isn't inherently bad and if you spin more than mash, twisting is greatly reduced.
The Advent X shifting is high quality overall. It's been around for 4 or 5 years and is common.