Mindless data dump that results in a terrible and very ignorant article.
I never heard of that website and I guess there's a really good reason to.
1 - The SteamVR test obviously didn't have the "-multigpu" command enabled, so he's probably just running AFR forcing both cards to render two viewpoints each, which is a terrible idea. Using 2*290X, I get the full 11 score with no drops using the multigpu command, and I doubt two RX580 would get him less than that. A 2-minute google search would have given him that information.
2 - Ashes of Singularity supports explicit multi-GPU and
it gets very good scaling on both brands, but explicit mGPU means he'd have to
disable Crossfire in the settings. The guy didn't bother doing a 1-minute google search before doing the tests, so he gets zero scaling.
3 - Doom doesn't support mGPU in Vulkan. A 30-second google search would have told him that.
Let's just spend tens of hours testing 20+ graphics cards to reach that conclusion instead.
4 - Deus Ex Mankind Divided
gets ridiculous scaling using DX12, but
let's just test it on the DX11 runtime that doesn't support mGPU, because reasons. Or rather because the guy lacks google search skills.
What really bothers me is the guy somehow has access to a myriad of graphics cards but uses them to make largely ignorant articles.
It's not like multi-gpu is great for everyone nowadays. It largely depends on what games one is willing to play, really. Two RX580 at their current prices are certainly not worth it IMO, but if one could get two 8GB RX480 for $180 each (like I got one a couple of weeks ago), or even two 8GB RX470 for $150 each using rebate promotions and the like, then it's definitely worth a look.