After my initial two attempts at soldering these boards I was confident I would end up with a higher yield rate >50%.

Apparently I was wrong.

After soldering 11 boards I ended up with 2.5/11 working ones which is a yield rate of ~22%.
The .5 had problems with the level shifter but is otherwise OK.

Lessons learned:

• soldering 0.4mm pitch WLCSP is hard
• proper stencil alignment and paste application is mandatory
• proper soldering equipment is a must (hot plate + hot air is too inconsistent)
• blaming the equipment for your mistakes is always possible

In short this means that these boards will not be available in my tindie store for a little while. I will have to fix the aperture for the stencil in some places and get a proper hot air oven (probably a modded toaster).

I am still happy the layout of the board was correct on the first try and, as usual, I learned a lot in the process of breaking things. I have also added a picture of my testing jig for your enjoyment.