The design is debatable, but it is normal for a raspi-based thing.
OpenWrt has the driver for the LAN7800 ethernet enabled in kernel config, so this dual gigabit board should 100% work out of the box already, if there is an image for the new Raspi compute module at all.
As long as you are using their own OS/firmware, yes. If you use a different OS, no.
It's a USB 3.0 gigabit adapter on a USB 3.0 port, so while it's not amazing it should be true gigabit at least and not a scam like with older raspi that had """"Gigabit""""" ethernet aka a gigabit adapter on USB 2.0 that can't run it at anywhere near Gigabit speed.
Although it's slightly debatable where the advantage is with this new carrier board compared to the option of taking a normal RPi4 and attaching a second USB3 ethernet card. Yes, the form factor is neater, but from a functional point of view it doesn't really provide any advantage, but a quite different price tag. Personally I'd rather put up with the ugly USB3 card hanging (hot glue to the rescue) from a normal RPi4, than buying this thing…
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Disclaimer: So far I've avoided the RPi universe myself, as it simply was unfit for my needs before the RPi4 and still has a few warts too many for my taste, even though it has gotten some unique selling points that trigger my interest.
this is meant for the kinds of customers that buy a SoM (the raspi module), so a more businness-ish embedded-ish segment that likes to use raspi ecosystem but does not want the product to look like a raspi in a custom case.
Somewhat to my dismay, it's a thing to develop embedded prodcuts on a raspi and then when it's time to actually market the thing they build they just transition to the compute module + custom base board instead of shotgunning hotglue around to stick the board and cables inside a box.
I do understand that, but I'm merely looking at it from the tangible-benefit-for-the-enduser point of view. Without a doubt it's nicer to look at (better to integrate, better mechanical stability), but from a purely functional ("I want a 1 GBit/s capable router able to do SQM at line-speed for my home") point of view, there is no (hard-) advantage with this, just a considerably higher price tag.
no-usb version (pci-based) released https://www.cnx-software.com/2021/03/27/raspberry-pi-cm4-iot-router-carrier-board-mini-offers-dual-gigabit-ethernet
openwrt - recommended
I was in the same camp, but I bought a raspberry pi400 recently, and I have to say, I really like that unit... (I have not tested it as router though).
Regarding routers, I wonder about using a compute module with the matching IO/board and use the PCIe slot to add a NIC, potentially an intel multiport NIC (probably not, still have a single port intel NIC sitting in a box somewhere) 
Which would be a poor choice hardware-wise given the rather lackluster connectivity of the RPi4 SoC.
Honest question, why is the PCIe slot available in the Pi4 CM/IO board combination not sufficient for a dual gigabit ethernet NIC? A PCIe 2.0 1x slot with 5 GT/s (per direction) should be sufficient for Gigabit ethernet's expected ~1GT/s per NIC, no?
Single port will probably work however multiport will be much of a stretch at least looking att Jeff Gerlings results. It can barely do 2.5Gbit with jumbo frames less than 2Gbit with.
This also seems to be in line with his testing of HDDs (bandwidth)
Mmmh, he references an earlier attempt where he transferred 4.15 Gbps through a 4 port intel ethernet NIC... Extrapolating from that I think two 1 Gbps devices should work out, barely.
I know that you seem to dislike the Pis (or probably rather consider other contenders in the same price bracket better/preferable) and you probably have good reasons for that, but I am unsure whether for the specific use-case I laid out your presented rationale is all that fitting.
Multiport would at least scale up to 4-ports which clearly wouldn't work, at least not to their full potential.
The Pi have their strengh but they're a poor choice for networking and its one of the reasons you have alternatives such as Marvell and solutions based on Rockchip SoCs instead which are better suited.
https://wiki.dfrobot.com/Compute_Module_4_IoT_Router_Board_Mini_SKU_DFR0767
The CM4 router is compatible with most systems that support Raspberry Pi 4, but most Linux distributions do not optimize the network device scenarios. So OpenWRT is recommended.
yes, but out-of-stock and 50% more expensive (45 vs 30$) no doubt due to chip shortage crisis...
yes,
for now i am forced to get their other cheaper single lan io board, until the other available
PiTray mini for Raspberry Pi Compute Module 4
https://www.dfrobot.com/product-2196.html
what's the PiTray mini's advantage over rpi4 standard (as both feature single lan)?
it a quite small and cheap board
but if you add up the cheapest cm-lite you surpass the rpi4's price and you get a lower spec (1gb vs 2gb of ram and no USB3 !) sbc with a need to build a custom case & cooling - no thanks
yes, indeed
here we dont get the Pi stuff at the actual prices it almost double in some cases
here is another possible option but you will have to make the board add PCI adapter