I use a 2018 Mac mini with an OWC Helios FX enclosure with a stock AMD Vega 56 GPU. My workload is some light gaming (Mac only no windows/boot camp) and 3x4K displays in scaled modes for web/software dev.
The mini itself is compact, fast, and generally great. Handles VM based dev workloads well. It’s small and quiet (not always silent though).
I used the machine with just the built in integrated GPU for a few months. For gaming it was bad. For lots of 4K screens it was... doable, in the sense that you could actually plug them all in, but UI effects would lag and it felt weirdly disjointed. Some tasks would be very fast then hit a GPU related thing (scrolling, OS animations, etc) and lag, stutter, making the system feel slower than it was.
Got the eGPU and it was 100% plug and play. Works beautifully, completely fixes all the UI issues, games generally run great (as good or better than my previous hackintosh with the same GPU in a direct PCI slot)
I’ve run into a few very small issues. USB-C connector tolerances are not always tight enough to reliably move equipment while it is all on, so sometimes if you grab the mini and drag it around (maybe to position it to plug in another cable) the GPU can disconnect. This generally falls back “gracefully” to the internal GPU but can be a bit annoying. You also have to think a bit more about heat management in your space. The GPU and mini all get hot when under load, you can’t stuff them in an unventilated hole. Overheat behavior is generally that the GPU shuts off and needs to be power cycled.
Compared to an iMac... The iMac is probably a better standalone deal and experience. However, in my case I have many machines and many displays. Mini+GPU lets me share generic displays across multiple machines in a way that an iMac simply can’t. Especially if you have existing windows gaming hardware where you can “recycle” the previous gen GPU in the Mac it can be way more cost effective. The GPU I am using is more powerful than you can get in any mac other than the Mac Pro.
Vs the Mac Pro... well the Mac Pro is better, but costs 4-a zillion times as much. kind of different ballparks. If you want a headless desktop and don’t need the Mac Pro crazy, a mini+GPU is a good option.
As a budget option? Mini+enclosure+GPU isn’t particularly cheap. My setup was ~2k without displays and I have a fairly simple spec setup (512GB, 16GB ram, slowest 6 core).
Where it shines is in modularity with other equipment. I can share the eGPU with laptops, share the GPUs with other windows machines, and share displays with other machines, in that environment it is cheaper and more flexible.