See also: Understanding Thunderbolt 3 Bandwidth.
Thunderbolt 3 can “daisy chain” devices”. Daisy-chaining means that one device can plug into the end of a string of devices, one after the other. Each device must be powered on for subsequent devices to operate, which is why a Mac with four Thunderbolt 3 ports (versus just two) is a huge plus—some devices can be turned on or off independently of others.
USB and USB-C cannot daisy chain at all; they must plug into a star topology, e.g., directly into the computer or into the older and slower technology of a USB 3.1 hub (there is no such thing as a USB-C hub, as it is part of Thunderbolt 3).
There is another Thunderbolt 3 limit: a maxium of six (6) devices per Thunderbolt 3 bus (and good luck not having cable-jiggle problems with random disconnects). From Thunderbolt 3 Technology Brief:
If each consecutive device is a Thunderbolt device, up to 6 devices in a chain can be supported. At any time if a USB or DisplayPort device is plugged directly into the USB-C connector downstream from any device, then the Thunderbolt daisy chain is terminated at that point.
This 6-device limit is not an issue with the 2018 MacBook Pro or 2018 Mac mini or 2017 iMac Pro since those machines have dual Thunderbolt 3 busses supporting up to 12 devices. But the 2017 iMac 5K has a single Thunderbolt 3 bus, and making it much worse in a practical sense when juggling devices, only two ports.
Which leads me to: