it's early to tell; multi-gpu setups lately have performed very close to what a single card can do .. it's very rare to see a game totally not supporting it, at most, there might be some glitches in the early drivers.
It's not rare at all to see games not supporting it, at least at launch. Overwatch, fallout 4, the division, stellaris did not support crossfire at day 1. Overwatch got it a few weeks after, but its really buggy. The division and fallout 4 got it 6 months after release. Stellaris still doesn't work with CF.
It's extremely common to see games not supporting multi-gpu properly.
And this isn't even getting into half the games that "support" it but have never ending bugs. Witcher 3 supports Crossfire and scales well, but any time you stand still you can see textures flickering in the background when Crossfire is on. This is on a mature title. The division "supported" crossfire in that you could turn it on and get scaling, but it had nauseating disco flashing textures everywhere.
The only game I've purchased at launch that has had perfect crossfire was Star Wars Battlefront