Been building SLI boxes for years and as of yet have yet to encounter a problem .... have three builds here (560 Tis, 780s and 970s) all worked day they were built and the 560 Tis are still working just fine 6 years later. In each case, they outperformed the x80 from their generation and by as much as 40%. Yes, have been instances where a game does not support SLI but overall we get 40% better performance for less money spent.
That being said, I don't know that I would SLI the 10xx series. Where scaling on the 970s in techpowerup's 23 game test suite averaged 70% (40% faster than the 980), with the 10xx series we are jut not seeing that. At 1080p, it's just 18% ... at 1440p, it's just 33%. As to why this is we can only guess but as long as AMD has nothing on the market to compete with the 1070, the only card sales that are negatively impacted by improved SLI performance would be the 1080 and 1080 Ti ... and why would nVidia want to cannibalize sales of heir big money cards ?
As for mixing cards, theoretically its supposed to work. The reality is, when it doesn't, you have 0 support options with each tech support guy blaming the 'other card". The thing is, the 1070 is more than fine at 1440p and way overkill at 1080p. The 50% SLI scaling at 4k is somewhat exciting but 4k needs twin 1080 Tis ... and until those AU Optronics screens from Asus and Acer drop next month, I just don't see 4k as all that interesting.