High Sierra tuned in native support for eGPUs, but recent updates on eGPU support on older Macs are looking grim. If you own a Mac that does have a USB-C port, (or any plain MacBooks) it is very likely your computer does not support Thunderbolt 3. Any Mac with Thunderbolt 1 or 2 ports are now less likely to support eGPU even with High Sierra or higher.
As of 10.13.2, eGPU support on older Mac is already deteriorating. The community is suspecting HDCP compliance on Mac side could have changed to TB1/2 hostile; if you are using TB3 to TB1/2 adapter, iTunes would produce HDCP error like symptoms. Again, there is no absolute guarantee here. Maybe Apple has actually decided to rule any kind of adapters out of HDCP, or maybe Apple is slowly killing off TB1/2 eGPU connectivity. Should Apple decide to discontinue eGPU supports on Thunderbolt 1 and 2, this may as well infuriate Mac Pro users with TB2 ports only.
It should also be noted that as of 10.13.3, eGPU is rendering the system incredibly unstable. One of the many symptoms I personally have includes not being able to boot with the eGPU connected. It must be booted up with internal graphics, through the HDMI port on Mac Mini (one from the review, yes), and then power on the external graphics, log out, reconnect the HDMI through the chassis. Be as it may, the beta status of eGPU cannot possibly excuse the increasing number of glitches.
In the upcoming 10.13.4, it appears Apple may be planning to phase out TB1/2 entirely. Currently released as public beta, since PB2 to be exact, older Macs connected through Thunderbolt Adapter experienced kernel panics when eGPU is connected. In PB4, external graphics card is detected without any major issues, however, the machine cannot use the graphics card to compute. Oddly enough, this is not the first time Apple broke eGPU support for Thunderbolt 1 and 2 and then later fixed it. That being said, it is possible that Apple is dropping the entire pre-TB3 support —which could be easily mitigated by the eGPU community much like before— or else, such as TB1/2 support has been assigned lower priority since the kernel panic patch. There is nothing solid at the moment, aside from the obvious. Not updating to 10.13.4 is the only recommendation I can hand out.
To summarize, if you are on eGPU, hold out on public beta programs until the native eGPU support sheds the beta sign. If you own TB1/2 machines running with eGPU, 10.13.4 could hit with some bad news; you may end up holding out on updates until Apple reintroduces TB2 compatibility. Ultimately, I can already see Apple laying out features that will be newer standard exclusives, —e.g. certain higher end graphics card only works through TB3— so it would be a safe bet to invest in USB-C/Thunderbolt 3 machines. Hopefully Apple will bring rest of the Mac Desktop lines up to the speed before the end of 2018.