I am running an RX 590 in the OWC Helios FX 650W eGPU enclosure. In Geekbench, I am getting roughly 3X's the performance of the internal Radeon Pro 560 in my 2017 MacBook Pro 15". So far, I'm happy with my purchase. I got the whole setup for just over $500 at Microcenter. The numbers were 46,000 for the Radeon Pro and 129,00 for the 590. The only thing I see is that the card is identified as an Ellesmere Prototype. So, MacOS as of 10.15.5 does not recognize the card correctly.
This not expected identification doesn't look to be an issue in Mojave, while in High Sierra the RX590 is really slow. "Ellesmere" is correct by the way, the RX590 chip is Ellesmere. There are ways to change this ID-String, @mac_editor described this once. You may find this in this forum.
I finally tried to add an external monitor to my setup. However, the external monitor remains blank, even though, the MacBook Pro recognizes it, and it seems to be receiving a signal. I have an ASUS gaming 4K monitor connected to the HDMI port on the RX590. Should I be using a different port?