How strong is the A380 fuselage-wing joint?
how could it be trusted?

Like all aircraft, type certification requires that these be tested. I think the wings are tested to 1.5 x their load limit.
A380: STRUCTURAL STATIC TESTS

The A380’s structural static tests on began in November 2004, in preparation for the first flight clearance.
The tests included: Flight Test Installation (FTI) calibration test, maximum wing bending at limit load, ailerons and spoilers functioning test during max wing bend, fuselage pressure test, and fatigue tests and flight cycles simulation.

They place an airframe into a test rig and increase the load on the wings until the wings break. The empirical evidence is compared with the predictions made by mathematical models.

Airbus A380 wing test

Airbus A350 wing test ... (A350 torture video)

Boeing 787 wing test