The TL;DR version would be, it does exactly what you hope it would, acting as keyboard AND mouse (with Fn key and arrows). Recognized on MacBook and Raspberry Pi without issue using example code from the maker. Gold plating on the holes grabs the solder and spreads it nicely, making it relatively easy to put together.
The longer version would be, this is absolutely amazing, I love it! It is so satisfying typing on those little clicky buttons! I'm using this, along with the plans on Thingiverse (https://www.thingiverse.com/thing:3209958/) to make the same portable Pi. Since it's the same width as the Pi 3 B+, it seems like an obvious choice for anyone who wants to build a hand-held. All of the keys work as expected, and the mouse functionality is surprisingly good considering you're moving the cursor with the arrow keys. There are a lot of retro games that I haven't really spent any time with because they require a keyboard, so I think it will be fun to have a handheld with one built in.
One caveat, I wasn't really sure how to attach the Teensy, so I used header pins going through both boards. Once the pins are soldered in place, it is REALLY hard to get the black plastic piece off without damaging anything (I knocked one of the SMD parts off near hole 19, but it didn't seem to matter). I'm not sure what the best way to do this would be, but, don't solder it with the black strip touching either board or it'll be a pain to get off later.
10/10, would highly recommend this keyboard PCB!