Do docking stations work fine?

Do docking stations that support DP over USB-C work fine on Qubes? I want to connect 2, maybe even 3 external monitors to a laptop.

I have some noname thunderbolt dock station with HDMI and it works fine in Qubes OS.

Thanks. However, the docking station would have to be permanently assigned to dom0, is that right? If a laptop has only 1 USB controller, that would mean basically no protection gains from using sys-usb.

Not sure about USB dock stations, but thunderbolt dock station works separately from USB and you have both USB controllers and Thunderbolt controllers in your system. You can passthrough USB controllers to sys-usb and leave Thunderbolt controllers in dom0 then if you connect USB device to the port then it’ll be using USB controller and will be connected to sys-usb and if you connect thunderbolt device to the same port then Thunderbolt controller will be used and it’ll be connected to dom0.
But USB devices connected to Thunderbolt dock station will still be connected to sys-usb. Only PCI devices from Thunderbolt dock station will be connected to dom0 e.g. PCI Ethernet controller.

usb usb-c dock adds usb devices to sys-usb but the hdmi output works fine

But if I connect a HDMI monitor to a docking station that is itself connected via USB to the laptop, doesn’t that make the HDMI monitor a USB device that needs to be attached to dom0, and not sys-usb for the monitor to work?

If it’s using Alternate Mode then it should connect your display directly to GPU below OS level:

But the hardware should support this Alternate Mode for it to work.

So basically, if a device states that it supports DP over USB-C, then if I connect a monitor directly via USB-C (both ends), it should behave exactly as if I used a HDMI or DP port?

Yes. But both devices and cable should have the support.