I've found with CF when it works, it works really well. You get incredibly good performance and scaling. When my 2 x 290s kick in I've got performance between a 1070 and a 1080. When it doesn't work, it really doesn't work. Most common is that it "works" in that Crossfire can be enabled on a game, but you get horrible artifacts. Flickering textures / objects is the most prevalent bug you'll find. It's nauseating so you have to turn it off. These days its because games just aren't all that good with AFR, and only good PC devs put in the pointers and vendor specific code to get both SLI and CF working well out of the box.
Lots of 2d primary games absolute crash and burn with CF, e.g. 4x games and strategy games with lots of UI. I can't play any Paradox grand strategy with Crossfire, not that I need the performance. But you have to fiddle with turning it on and off a lot. It's much nicer now that they give you a per-game profile option to turn of CF or not (and it actually works now).
You'll get weird, inexplicable bugs too. In the Paradox games I mentioned (Hearts of Iron, Europa Universalis, etc.) when in crossfire there is weird shadowing, and certain 2d UI elements appear for half a second and flicker, and certain other UI elements appear totally normally but can't be clicked. Imagine your frustration when you can't click to move units for no apparent reason... Took me a day of googling to figure out it was a CF related bug.