Interesting! Was wondering about this recently when shopping for a USB-C hub, and couldn't find many that supported 4K@60hz. Ended up getting one that does 30hz because my Display at home is only 1440p.
I have used Displaylink adapters before (have one floating around somewhere), and was not super impressed (my machine was a bit flaky when using it), although I did have 2 external displays going on my Macbook at one point (this was before the days of multiple Displayports on Macbooks), which was kind of fun.
I went through 3 of them before finding one that worked (the first was buying before I knew the issue existed. The second was being impatient). I'm using this one on a Macbook Air. (https://www.hypershop.com/products/hyperdrive-duo-hub-for-us...). I'd prefer one that used a single USB-C connection (assuming that's possible) and that used a cable so it was useful for more than just the couple of devices that have 2 USB-C ports exactly that far apart but at the time, a year ago, that was one of the few choices. The good thing is it does work and I have it on 4k@60hz
I have used Displaylink adapters before (have one floating around somewhere), and was not super impressed (my machine was a bit flaky when using it), although I did have 2 external displays going on my Macbook at one point (this was before the days of multiple Displayports on Macbooks), which was kind of fun.