Yes, I’m using it on a Mac Pro with an RX580X and two RX5700XTs (I got the year’s free Enterprise license). It’s fairly stable these days – with the usual caveats about GPU renderers, when you hit VRAM limits. If you’re doing small-scale stuff, then you should be fine.
Compared to C4D’s Physical renderer… well, there is no comparison. I can render a scene with billions of instanced polys, volumetric lighting and GI in seconds instead of many, many minutes. The denoise function also works well enough to knock even more time off, and is great for things like OpenVDB smoke and clouds. As with all modern, fast renderers, the realtime IPR is a life-changer.
Octane’s material system is easy to understand (either with layers or nodes), and it’s quite feature-rich now, for things like render passes.
And because Octane is rental, you can drop $20, check it out and then cancel if it’s not to your liking.
I’m not really up on eGPU options – check out https://egpu.io
I will just add that yes, these AMD cards aren’t a patch on Nvidia’s, but this system is easily fast enough for my needs. I hope to drop in some 6800XTs at some point, but the biggest plus would be more VRAM.