The OWC unfortunately has had some terrible reports, and what with the limited power, both contributed to making many users skip buying one. While not perfect, the CalDigit has better specs overall – obviously likely as it's the latest, being released recently.
Seems that the only reason to get the OWC is FireWire (without comparing benchmarks, latency measurements, audio capabilities)
Many would love a switch type thing so they could plug one (or even two) LG 5K displays into two TB3 Macs, and easily flick between them. As the trouble with these displays is they only have the single TB3 input. (Even if you couldn't use the 3 5Gb USB-C ports on the displays back.)
The LG 5K display can't have two Thunderbolt 3 ports because one Thunderbolt 3 port connects to the computer, the other Thunderbolt 3 port is for one of the LG's DisplayPort 1.2 inputs and the DisplayPort output of the Thunderbolt 3 controller is for the LG's second DisplayPort 1.2 input. Two DisplayPort 1.2 inputs are required for a 5K display (just like the discontinued Dell 5K display except the LG 5K receives both DisplayPort 1.2 signals over Thunderbolt 3 from the Mac). Basically, The LG 5K is wired like a Thunderbolt 3 Dual DisplayPort adapter (but also uses PCIe tunneling for the USB functions of the display - a PCIe USB Host controller is connect to the PCIe interface of the LG's Thunderbolt 3 controller).
Your last two paragraphs are interesting, love to hear some further (understandable) explanation/examples on those points.
The Thunderbolt 3 controller has a 4 lane PCIe 3.0 interface. In a Mac, the Thunderbolt 3 controller's PCIe interface faces upstream to one of the Mac's PCIe root ports (link width is 4 lanes). the Mac's Thunderbolt 3 controller's Thunderbolt 3 port has a downstream connection to a Thunderbolt 3 dock's Thunderbolt 3 controller's Thunderbolt 3 port.
A Thunderbolt 3 controller includes a built-in PCIe switch.
In a dock, the Thunderbolt 3 controller's PCIe switch uses a Thunderbolt 3 port for a 4 lane upstream connection to the Mac's Thunderbolt 3 port. The PCIe switch divides the Thunderbolt 3 controller's PCIe interface into 4 links, with width of 1 lane each. Downstream to each link is a PCIe device (USB host controller, Ethernet controller, etc.)
The Thunderbolt 3 controller's PCIe switch can use the Thunderbolt 3 controller's PCIe interface as a 4 lane link to a downstream 4 lane device (such as an NVMe drive) or a 4 lane slot (as in a Thunderbolt 3 expansion box like the Sonnet Echo Express SEL or SE I)
I don't know if there exists a Thunderbolt 3 device that uses the PCIe interface of the Thunderbolt 3 controller as a two lane link to a downstream device (or two 2 lane links to two devices). However, the MacBook Pro 13 inch has one Thunderbolt 3 controller where the PCIe interface is used as a two lane link to an upstream PCIe root port of the Mac.
In a 3 slot Thunderbolt 3 expansion box, the Thunderbolt 3 controller's PCIe switch uses the Thunderbolt 3 controller's PCIe interface as a 4 lane link to a downstream PCIe switch having multiple additional downstream links - one to each slot. The PCIe switch is usually one of the PEX 87xx variants. For example, the Sonnet Echo Express III-D (or the Sonnet Echo Express SE III) uses the PEX 8724 (PCIe 3.0, 24 lanes), configured with one 4 lane link upstream, and 3 links downstream (x8, x8, x4). The PEX 8724 can provide 6 links, so it could be used to provide 5 slots. In a Thunderbolt 3 dock it could allow for 5 PCIe devices.
Another example is the Amfeltec SQUID PCIe Gen 3 Carrier Board for 4 M.2 SSD modules. The x16 version uses a PEX 8732 PCIe switch (x16 upstream slot, x4 for each of the 4 NVMe downstream slots = 32 total lanes). The PEX 8732 can provide 8 links. The PEX 8733 has the same lane count but can provide 18 links.
So with a PCIe switch, you can add many devices (you just need space and power for them). There is a small amount of extra latency (but not as much as with the Thunderbolt 3 connection itself). The devices share the upstream link. There is a limit to the number of PCIe devices that a computer can support (each downstream link is a new PCIe bus - usually up to 128 allowed, or 256 on some computers). Each device takes I/O memory space which is also limited. A computer may reserve a number of buses and some I/O memory for each of the two Thunderbolt 3 ports of a Thunderbolt 3 controller.
As for USB, the OWC Thunderbolt 3 Dock uses two Fresco Logic FL1100 4 port USB 3.0 host controllers. The CalDigit might be similar. I was asking if anyone makes a host controller that provides more than 4 ports. If not, then they could add a USB hub internally which would behave like an external USB hub.
For example, the Anker 10 Port 60W Data Hub is a 7 port external USB hub (3 ports are only for charging), but internally, it uses a 4 port hub connected to another 4 port hub meaning 4 of the ports have slightly more latency than the other 3.
So with a USB hub you can add more USB devices. USB ports of a hub have to share the USB bus.
As for USB 3.1 gen 2, one of the most common host controllers is the Asmedia ASM1142 which provides two USB 3.1 gen 2 ports using a PCIe 2.0 x2 or PCE 3.0 x1 interface (same as can be provided by the Thunderbolt 3 controller). CalDigit uses the ASM1142 in some of their PCIe cards like the FASTA-6GU3 Plus. If they use this in their Thunderbolt 3 Dock, then they could have provided two USB 3.1 gen 2 ports (but may need a larger power supply or more space?).