I’ve tried the HDMI out, a Blackmagic 4k Mini Monitor, and a Blackmagic Ultrastudio 4k Mini. All 3 work fine.
Blackmagic Ultrastudio 4K Mini - looks cute with the little screen, but wish it was either smaller or was a 1u so I could rack it handsomely. The 1/4inch left and right audio outs are helpful to feed a Mackie Big Knob. Frees up a PCIe slot in your Mac Pro if you are tight on space. Every once in a while I do get weird distortion in Spotify music whilst GPU is under heavy load…no idea if that is from monitoring all audio from the Mac Pro through the card or not. I can’t remember if you can monitor video only and use the 1/8th inch mini jack from the Apple IO card to monitor audio or not. I think you can, but its been a while. The Thunderbolt cable size limitations on the Blackmagic Ultrastudio 4k Mini are a pain for me, but, that’s my room/setup. Plus, I may be wrong, but sometimes I have to reset my PRAM because I swear a Thunderbolt device on my machine is acting up and the Mac Pro thinks its a system disk. But that could be another CCC bootable clone acting up on a SoftRaid volume I’m running.
Blackmagic 4K Monitor Mini - super cheap, love it, but you do need a de-embedder if you want to monitor audio from the card. I think I’ll revert to this to simplify rack space. But of course, it does use a PCIe x8 slot which is valuable real estate. I have a several SDI devices on my desk so its kinda a no brainer to just run an SDI from the rack to the desk and deal with scopes/audio monitors and an Atem mini with micro SDI to HDMI converters there.
Don’t forget about the whole audio monitoring thing. It gets annoying fasts if you are wanting to monitor audio from 2-3 devices, have 1 set of speakers, have a microphone, etc.
If you decide to go with the Blackmagic Ultrastudio 4K Mini, I’ll sell you mine and a Mackie Big Knob for a deal.