I recently purchased a "20 Slot PCIe 2.0 to PCIe 2.0 4U Expansion System One Stop Systems" from eBay. The goal was to get 8 or more BMD Intensity PCIe (Pro) cards working with a separate CPU box.

Here is my progress.

The 4U unit looks to have cost about >$4K in 2010 and there are a few left on Ebay. Depending if the host adapter is included, they will go for $500 w/o cable and adapter to about $1100 if including the host adapter card and interface cable. If you do some searching, a host adapter and cable can be had for about $200 or less. The ebay ad indicates these were spec'd by NASA.
They have twin Xeon procs on the mobo and 3 mega fans inside.

So how did it fare? It worked perfectly. I tested it with 15 cards installed. Though I had only 8 cameras hooked up, it switched smoothly between them all. The main CPU has a 4770K proc and a Radeon 6850 for video card. All inputs were via HDMI at 1080i, 59.97.

With this setup a relatively small motherboard (and machine) can be used since all that is needed a slot for the video card and another slot for the interface card. As you may have found if you begin to load up the PCI bus with multiple interface cards, the video card slows down as the bandwidth is shared with the other cards. This also depends on the mobo, but a majority work that way.

Latency was less than 20ms. A faster video card could bring it down is my guess.

BMD Intensity cards. In 2006 the "Intensity" cards were introduced without analog inputs. Then the "Pro" version came out a year later with analog inputs. Fast forward to 2015. I just picked up 8 regular "Intensity" cards from eBay. Updated the firmware and now they work like new. These were $50 each! Who needs analog inputs? Shotgun mics are attached to a few cameras so the A-D is done at the camera and both audio and video is input through one HDMI cable. No extra XLR runs or stand alone mixer is required! Next is to add a nanokontrol2 for easier panning and mixing the levels and less screen space taken for the vmix "digital" mixer.

I am very happy with this setup and can live mix 8 or more HD inputs all for less than $1500.
Maybe 2 of the YUAN x4 cards would have worked but?

BTW I have few extra BMD Intensity Pro cards I no longer need and will sell them. Message me if interested.

Also very happy with Vmix!

The interface card uses a pcie x8 slot so if the mobo would support more than one x8 including the requirements of the video card then it could work. Most mobos split the x16 slot into 2 x8s and then again if more card are put in.

You need more than 19 inputs?

Here's the data sheet. www.onestopsystems.com/sites/default/files/pdf/176-4u_vl_enclosure.pdf

The unit on ebay is the OSS-PCIe-BP-2022, PCIe 2.0 20-slot backplane with 1 SHB target slot, 2 PCIe2 x8 and 17 PCIe 2 x4 (20-slots)

One slot in the box is used for the I/O card or "Slave (Service?) Host Board" SHB, the remaining 19 slots are spit up and 17 being PCIe x4, and 2 are PCIe x8.

My SHB and Host interface Board (HIB) cards are PCIe x8's so they are rated a 40 gb/s. So it looks like it has the capability of a Thunderbolt v3, with 19 slots for cards. If the i/o card were set up for PCIe x16 then it looks like 80 gb/s. Hard to find a mobo that would support 2 simultaneous x16 slot though, one for the HIB and one for a video card.