There are many things that come with running more than one GPU, it's not just limited to the game not working at all.
The big question of course is your tolerance levels and how much you are prepared to put up with.
I ran SLI from 2007 all the way up until about six weeks ago and whilst for the most part it did what it should have it bought problems too. As I said, in Dying Light I would get great FPS @ 4k but with that came issues. When things were moving very quickly it would become stuttery just when you don't want it to. I also had the issue I described where the entire game would slow to a crawl and that would last anywhere between around five seconds and thirty seconds.
It was a well known issue, and IIRC it was fixed but it did take away my enjoyment of the game until it was fixed.
Witcher 3? hah ! that was just a pure mess in a dress on both Kepler and SLI. At first it was stuttery and I couldn't even get 30 FPS at high settings, then with the settings lowered to medium (where tbh it looked like 1080p) it flickered like a pig because apparently there was an issue with SLI and Gsync.
So there you have it, the 'I' word. Issues. All of which need to be suffered, overcome and waited on. Issues that don't exist when you are only running one card.
It was brought up lately that only around 300k users worldwide used more than one GPU. I would hazard a guess and say that even if those numbers are wrong the people who do run more than one GPU are very, very much in the minority.
As such the companies who own these technologies do not put enough time, money and effort into them and this is why you get issues.
Even when SLI does work properly and to its full capacity there are always tiny niggling little issues that come with it.
And Crossfire? well that is considerably worse. AMD cared so little about it that for years it was completely broken and didn't work properly. Even now, after they were caught, they still don't put enough time and effort into it. Why? all you need to do is look back and look at how few people use it. Why put time, money and resources into something that hardly any one has?
All of this shows when you get down and dirty and actually play games. None of the issues of a multiple GPU system show their heads because you're not taking the risk any more.
I promised myself a Fury X. I got one.
The next month I was dead set on getting a Fury to go with it. Instead I got a new coffee machine, 200 pods for said coffee machine, a 480gb SSD, a very expensive case for my phone, etc.
The next month I promised myself a Nano to go with my Fury X for Crossfire. Then I find out that it won't work in Crossfire and AMD want you to offer up a kidney for it.
£550 or more to add issues to my PC? sorry matey, don't think so !
I've been told elsewhere that Witcher 3 still does not work with Crossfire, so it would be £550 (or more gauging prices !) for nothing but crap.
Not going there. This is the first time I have seen completely issue free gaming in about eight years. I'm not really wanting to go back tbh.