The most problems I've had with SLI were with brand new, just released games not having SLI profiles. Users often found a working combination of SLI bits for a game before Nvidia updated drivers with a profile.
Other than that the most problems I've had have been with Far Cry 4 which was completely broken on SLI upon release (everything turning black at night, flickering environments etc) and with the 1.9.0 patch they've actually broken it again, now the environment flickers on SLI. SMAA on SLI is also still broken on that game (causes transparent ghost images of menus to stay on screen). For that particular game I didn't have any problems on patch 1.8.0.
So I would not say that SLI is without issues, but mostly it's a case of the developer not testing SLI properly (and lets face it, Ubisoft games are known for requiring a gazillion patches anyway) or Nvidia not providing a profile. I think this is overall quite minor though and haven't noticed any stutter etc issues.
By comparison AMD is doing a lot worse. Just this month they finally released drivers that add Crossfire profiles for Far Cry 4 for example, a game that has been out for something like 4 months now.
I've got 970 SLI myself. For about 500 euros more for similar performance the Titan X isn't all that exciting to me and neither is the 980 SLI. Since those two are so close in price, I'd pick the Titan X because it can run anything at 1440p really nicely when overclocked and you avoid any SLI related issues altogether.