I've had some more fun with the UltraFine over the holidays. And I mean it!

Connected it to a bunch of older Macs. (Next step is bringing up the eGPU setup again.)
Summary, from oldest to newest Mac:
27" iMac (Late 2009) with ATI Radeon HD 4850:
- 10.6.8: Monitor is detected; several resolutions up to 3840×2160 30 Hz are selectable; screen is black
- 10.7.5: Monitor is detected; several resolutions up to 3840×2160 30 Hz are selectable; screen is black
- 10.9.5: driven at 640×480 60 Hz; only 640×480 and 1920×1440 are selectable
- 10.10.5: driven at 640×480 60 Hz; only 640×480 and 1920×1440 are selectable
- 10.11.6: driven at 3840×2160 30 Hz; comes up in 1920×1080 HiDPI
11" MacBook Air (Late 2010) with NVIDIA GeForce 320M:
- 10.6.8: driven at 640×480 60 Hz; no other modes selectable
- 10.7.5: driven at 640×480 60 Hz; SwitchResX additionally lists 1920×1440 as "not active"
- 10.8.5: driven at 640×480 60 Hz; only 640×480 and 1920×1440 are selectable
- 10.9.5: Monitor is detected; OS says it's driven at 4096×2304 30 Hz, but it's black and GUI is sluggish on internal LCD
- 10.10.5: driven at 4096×2304 30 Hz; comes up in 2048×1152 HiDPI
- 10.11.6: driven at 4096×2304 30 Hz; comes up in 2048×1152 HiDPI
- 10.12.6: driven at 4096×2304 30 Hz; comes up in 2048×1152 HiDPI; brightness slider present and working
- 10.13.6: driven at 4096×2304 30 Hz; comes up in 2048×1152 HiDPI; brightness slider present and working
13" MacBook Pro (Early 2011) with Intel HD 3000:
- 10.6.8: kernel panic due to AppleIntelSNBGraphicsFB.kext
- 10.8.5: driven at 640×480 60 Hz
- 10.9.5: driven at 640×480 60 Hz
- 10.10.5: driven at 640×480 60 Hz
- 10.11.6: driven at 3840×2160 30 Hz; comes up in 1920×1080 HiDPI
- 10.12.6: driven at 3840×2160 30 Hz; comes up in 1920×1080 HiDPI; brightness slider present and working
15" MacBook Pro Retina (Late 2013) with Intel Iris Pro 5200:
- 10.9.5: driven at 3840×2160 60 Hz; comes up in 3840×2160 HiDPI (!); other HiDPI modes selectable via RDM or SwitchResX
Looks like I'm definitely exploring uncharted territory here (and loving it). It's very interesting that the minimum "working" version of OS X differs across machines/GPUs. Fortunately I have
one Mac that allows me to use the monitor on Mavericks LOL (but not at full resolution)! The built-in speakers are standard USB audio stuff and work in every configuration I've tried, back to Snow Leopard. I wonder why Snow Leopard, Lion and Mountain Lion (and to a lesser extent, Mavericks and Yosemite) don't seem to like the UF on the machines I tried.