After being held a few days in customs (no idea why, since the package was clearly never opened), the PCBs finally arrived, and as I promised to JLCPCB, I'm going to review them. On the first sight, they look great:

And I expect them to work very well, even given some really tight tolerances around the matrix controller chip. The outline is good, except for a tiny little notch near the top left corner, where the CNC machine probably started. Generally, I have nothing I could complain about.

However, just out of curiosity, I did a test that the people from Aisler taught me when we were comparing their boards with ones I had fabricated in other fabs. You have to hold the PCB against light and then you can see where they removed the soldermask completely, leaving naked FR4 underneath, to avoid covering the chip pads. It's a little bit hard to take the photo of it, but it looks something like this:

You can see four lighter rectangles around the pads of the chip. That's where there is naked FR4 on the top side. There is no such a thing in the original gerbers I sent them, they modified them to avoid problems. Maybe you can see it better with a magnification:

Of course this has no impact on how the PCB will work — it's just interesting to compare their process with that of Aisler.

All in all, I'm very happy with the PCBs, and you certainly can't beat them on price (though the shipping cost kinda offsets that). I will certainly order PCBs from jlcpcb.com again.