Nice to see. Disappointed to not see mic/headphone jacks on that carrier board (but that might explain the mysterios AN0, AN1 pins that are available on the CM4... interesting.
Packaging of eMMC is nice... I wonder if we will see it on a future RaspberryPi4 instead of having to wait for a 5 to include either eMMC or an M.2 connector to add you own NVMe drives.
If I had the physical space in some of my current projects (presently no), I'd use this compute/carrier board combo as a Pi4 substitute.
I don't mind the additonal cost. It's the 3X greater real-estate that's an issue.
I also adore the 4Bs mounting holes that setback from the edge with the USB and Ethernet ports... Nice for mounting on a backplane, and still being able to make cuts in a case and have those ports accessible... without having to create a unique mounting. ... However, it would be nice to access the power from that side, but alas there is no space..
That said, the IOBoard has the advantage of all externally accessible ports being mounted along one edge... USB, Ethernet, HDMI, 12V Barrel connector, SDCard (which would go unused with the eMMC option)
Hoping for a RaspberryPi4B+ to give us an eMMC, an external antenna option, a power header so we don't have to use the usb pwr... I use the DSI ports for video, so it's not a terrible thing that I cannot easily access the HDMI ports, nor the power.
Sacrifices either way. If it weren't for that size.. I'd be tempted by the CM4/IOBoard/Antenna option. (I literally can't fit it in my current projects).