Unfortunately the information you're after is buried in user manuals (if you're lucky) or the specification documents. That's your research to do.
Here is one example from Tyan:
https://www.tyan.com/Motherboards_S8020_S8020AGM2NR-EX
If you look at the spec sheet:
ftp://ftp.tyan.com/datasheets/DataSheet_S8020.pdf
on page 2 is a nice block diagram that highlights:
3x PCIe x16 slots are PCIe G3 x16
1x PCIe x16 slot is PCIe G3 x8
2x M.2 (2280) are PCIe G3 x2
Some of the USB3.1 are off the CPU, and others are off the X399 chipset.
The PCIe x4 slot is off the X399 chipset, and so on.
So from the block diagram you can work out if this board is for you or not?
PS. This is not a recommendation or endorsement of this board, just using it as an example. However it does seem to meet your needs other than coming with 10GbE onboard and SAS. (both easily corrected with add-on cards).
Re: M.2 only being x2 slots, note: PCIe G3 is 8Gb/s per lane (so 16Gb/s for a x2 slot). There is no reason you couldn't use M.2 / PCIe add-in cards either if this is limiting.
Another example is the:
https://www.supermicro.com/en/products/motherboard/X11SPA-TF
Under key features they clearly list the shared slot architecture, so nothing is hidden. You just need to do this research yourself (if these details are that important to you).
You need to understand most review sites are aimed at gamers (people with more money than sense) and are largely marketing sites for the manufacturers... Unfortunately sites like 2cpu.com have slowly died, and once highly technical sites like ArsTechnica are catering for the more mainstream users...