Those aren't supported natively via the chipset, these are implemented using extensions which result in poorer performance and eating quite a bit of the PCIE bandwidth available. The lack of USB 3.1 support is also the problem I have with Haswell-E, I have an X99 motherboard for my Broadwell-E setup and USB 3.1/TB is only available via the ASUS extension card which eats up 4 PCIE lanes and doesn't perform nearly as well as the "native" Skylake Z170 series chipsets.

As PCIE3.0 is not supported on AM3+ motherboards because this requires native CPU/Chipset support so you are overloading a potentially already contested PCIE (especially if you are running multiple GPU's) bus which has overall less bandwidth to offer which can yield even worse performance than what is available on X99 chipsets currently which lack some of the same features but at least support PCIE3.0.

That's correct of course. But, do you know of some benchmarks showing the real world problems those AM3+ boards can have? I'd really be interested in a proper source.

There aren't any "universal" benchmark since each motherboard will implement those features differently, different chipset, differently bandwidth allocated to it (USB 3.1 supports from 1 to 8 or 16 PCIE lanes IIRC) even things like the length of the traces can affect the performance.

So you'll have to check for benchmarks for individual motherboards by make and model, not all reviews cover USB 3.1 or NVME since many of the reviewers don't even understand that those features can behave substantially differently so googling AM3+ USB 3.1 testing or something would be your best option some reviews do include those tests for example: https://nl.hardware.info/reviews/6106/5/msi-990fxa-gaming-am...

Overall considerably lower performance and that's against older intel motherboards that do not have native USB 3.1 support either, mostly because of PCIE 3.0 support and substantially better PCIE latency over both the PCH PCIE bridge and the native CPU PCIE lanes.

Based on the numbers from this benchmark I can tell you that the USB 3.1 10gig performance on newer motherboards is considerably better than that, just by comparing my X99 (which is also tested in the aforementioned benchmark) to my high end Z170 motherboards with the same storage peripherals. Overall I'm getting slightly better speeds on my X99 setup than they had (probably slightly faster storage) but the Z170 offers anywhere between 20 to 30% better performance depending on the benchmark with the same SSD based external storage.