One issue is that, at least on cheaper mobos, these don't work as a "total bandwidth budget" situation. And, especially with newer generation PCIe, it can be a bit frustrating.

Many mobos will operate the available slots such that the total number of active lanes is split between them. But if you use older-generation cards, you'll only get a fraction of the available bandwidth because you're only using a fraction of their lanes, although the physical lanes are physically present.

What I'm thinking about is something like, say, a pair of Gen3 NVMe drives that are good enough for mass storage (running in RAID-1 for good measure) and some cheap used 10 Gb NIC, which will probably be 8x Gen2, all running on a Gen4+ capable mobo.

And, while for a general-purpose setup I can live with splitting available BW between the NIC and the GPU (I most likely don't care about my download going super fast while I game), the downloads will generally go to the storage, so they must be fast at the same time.

Those MBs are cheaper precisely because supporting this kind of bandwidth breakout adds cost (-> a fancier PCIe switch in a higher end chipset/southbridge). If you add the support to do this to them you end up with the more expensive motherboard. Some of the highest end motherboards actually have 2 chipsets/PCIe switches, more cost but a more bandwidth sharing for the same number of lanes coming from the CPU.

You can also buy external PCIe switches (just make sure you're not accidentally buying a PCIe bifurcation device). Most of the time it's cheaper to just buy the higher end motherboard though, e.g. I don't want to know what price "Request a quote" for this PCIe switch which can do x8 4.0 upstream and then quad 4x 3.0 downstream https://www.amfeltec.com/pci-express-gen-4-carrier-board-for... is. I do have a few 3.0 era cards which were more reasonably priced though https://www.aliexpress.us/item/3256801702762036.html?gateway... and they've worked well for me.