What kind of temps do those M2 drives have?
I was concerned about that too, since there is not much space between the m.2 adapters in this configuration (there was much more space in the old server board, as its PCIe slots were laid out with 16x and 8x slots in an alternating fashion always leaving a gap between them)
In practice - however - I have not had to worry.
The little fan on the board, coupled with the server case airflow across the aluminum exterior does a surprisingly good job for its diminuative size, and low amount of space for airflow.
There are only a few mm between each of those cards, but it seems to be enough for the little fans to pull enough air over the aluminum heatsinks.
Gen3 m2's tend to sit at idle at about 26C-28C
Gen4 m2's tend to sit at idle at about 35C-41C depending on the model
The two m'2's directly on the motherboard, with those low profile heatsinks installed idle at about the same temp as the equivalent gen4 models in the adapter cards, and I gave those their own little 40mm Noctua fan to blow directly on them because I was worried they might get too hot. (it's visible on the left of the big PCIe cards in the pic.
For comparison, the two Intel Optane 900p u2 drives over on the right in that makeshift 2.5mm drive cage I taped to the side using 3m mounting tape idle at about 40C.
Most of these nvme drives (15 out of the 16) are used in ZFS pools, so to give them some load, I did a ZFS scrub and monitored their temps. Load temps seem to go up 5-6C over idle.
So yes, the concern about the airflow over the m.2 drives was real, and I was concerned as well, but testing has shown it to not be a problem. I even considered using some adhesive backed little rubber feet between them to force them apart just a tiny bit to make sure the air could get in, but looking at the numbers, I just don't have any reason to mess with them. They are doing their job.
Those
Asus Hyper m.2 Gen4 cards may be consumer products, but as I understand they were based on enterprise designs. I was going to buy the famous Dell cards instead that are actually enterprise products, but when I was shopping for them years ago I couldn't find any in stock anywhere, and I was working against a deadline, so the Asus ones had to do. They are surprisingly well made for consumer products, and rely on PCIe bifurcation just like an enterprise model would, so the signal path is relatively simple. They are also fully powered by the slot, so no need for power cables.
I have been very happy with them, and wouldn't hesitate to recommend them even to enterprise users. Just have to keep an eye on those little fans. No idea how long those bearings are rated for.