Well, it caused me to be totally anal about the motherboard selection, which may not be a bad thing in the long run. My daughter has been running 2 x R9-290x’s in crossfire for about 3-4 years now on a 34" 4k monitor for gaming, not had any issues at all with Crossfire, always worked well for us.
I also had the two RX580’s kicking about doing nothing, these were two of the four that we spoke about at the TPR LAN event that had fan failure and got RMA’d. Two went back into the Etherium mine, the other two were spare…
Sarge
Because you have them already is a great reason 
People use GPUs to game? Whatever next? Maybe I’m looking at it from a different enough angle I hit different problems. Think I mentioned I started to dabble with competitive overclocking. There’s a lot of quirks when it comes to having more than one AMD card in a system. Often the driver wont remember the settings and I had a routine of what to change when, after ever single boot, as it crashed a LOT.
Interesting you mention 4k. I tried that with a single Vega 56 on FreeSync monitor, and I was struggling to maintain 60fps unless I dropped settings a lot. With FreeSync, found it usable down to about 50fps or so but it didn’t feel good below that.