The viewport renderer still outputs ERROR to the screen (with no log) if an eGPU is the sole rendering device selected to power the viewport and as such viewport rendering is quite slow.
RPR 1.8 v250
eGPU RX 580
eGPU WX 9100
MacBook Pro 2017: i7 3.1 with dGPU Radeon Pro 560
The 560 dGPU is the ONLY GPU that will operate at all on the Preview Renderer. (CPU works).
The production F12 render can be set to use eGPUs exclusively and works fine and very fast.
Ideally at least one eGPU should be set as the Render Preview device.
This bug has been present in the previous 3 builds.
Build v250 though is way way more stable than any previous build for me. I could happily live with the above bug because it is currently very stable. I don't know if this is a fluke yet. I will have to see how the next few days go before I get the champagne out.