The proof? Crossfire AMD requires you to download extra packages for extra games don't have the packages then it messes up a lot. SLI is known to be a lot more stable for a lot of games compared to Crossfire. It is subjective and opinion based but as you said Nvidia works with a lot of games etc which means their drivers and such are actually built around what the game will be doing. AMD sell their better cards a bit cheaper than Nvidia for that reason making their Crossfire a cheaper alternative but with the extra packages its a bit more hassle tbh. Im not saying Nvidia performs better for the cash, they both work well. I try to be open now on the subject. But Nvidia drivers and SLI generally work better for the games etc.
And that is a lot... and Im not reading over it because it probably looks god awful but hope you get the point xD