Only on M1. On M1 Pro and Max you’ll be fine driving two 4K DP screens off the same TB4 port via hub, or via two cables if you prefer.
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Yep. Looks like the vendors’ literature (with the partial exception of CalDigit) has yet to be updated, but I’m assuming the same: The limitation was on the original M1 chip, and has been addressed with the M1 Pro/Max.
Look out for, and avoid, products that use daisy-chaining of DisplayPort (many monitors, some cheaper plug-in adapters).
This requires support for something called Multi Stream Transport (MST), which macOS* does not have.
In the small print you’ll usually see it explained that, with a Mac, the two external displays will be mirrored. But it’s easy to miss that detail in the regular cluttered Amazon product listings.
[* Reports so far indicate that Monterey hasn’t added MST, either.]
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Yes, but keep in mind that USB, Thunderbolt and DisplayPort are different technologies, even though they may all use the same connector.
USB is not a video interface, although USB-C has an alternate mode where the connector can carry DisplayPort video signals.
Thunderbolt also is not a video interface, but there are two different ways that a Thunderbolt interface can carry DisplayPort video data. One is in Thunderbolt mode, where up to two streams of DisplayPort can be carried alongside other Thunderbolt data. And the second is an alternate mode where the entire connector only carries DisplayPort video signals (equivalent to USB-C’s DisplayPort alternate mode).
If the port (whether USB-C or Thunderbolt) is running in DisplayPort alternate mode, then the port will only output DisplayPort data. In order to support more than one display, the computer must support MST.
If, however, the port is running as a Thunderbolt port (meaning the other end of the cable is either a Thunderbolt hub or a Thunderbolt display), then the computer can send two streams of DisplayPort data. These can be broken out by a hub (with one or more video output ports) or by a downstream Thunderbolt port built in to a display (the display consumes one stream and passes the other downstream).
The Benq article describes both methods. Only one of them (the Thunderbolt method) works on Macs.
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So Amazon US finally got some stock on this hub. Now $250, and as of this writing only 17 left in stock. Well…16. I bought one. 
And they’re already sold out again. It’s almost as if they were being assembled by hand at a rate of about three a day. 
If it were me, I’d be raising prices even more. 
Interesting. Maybe CalDigit was testing the price increase with the first case they released to Amazon, because now it’s back in stock, still at the new price, and no stock limitations appear on the page. (This is pure speculation–I have no idea how Amazon and vendors work out this sort of thing.)
Yeah, managed to snag another one. This time for myself.
It’s a bit odd how they do this. They don’t have them in stock on their own store where they still advertise $229. On Amazon they are now again selling a couple at $249, but it doesn’t even show up on their Amazon store page. You need to actually search for it by name. 
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