I have never experienced "hassles" with multiple cards provided I didn't use low end cards (top 3 tiers only) and, at least w/ Crossfire, not more than 2 cards. I mean yes, you have to sometimes wait a week for an SLI profile and 3 - 5 weeks for CF but as my kids don't buy games right away ... (wait for those half price sales) ... it don't matter.
However I don't see the Fury X at all competitive with the 980 Ti..... especially overclocked (30% difference)
The 390 CF is competitive with the 970 in SLI ... but not when overclocked. However, depending upon what games ya pick.... you can make either one a winner so what you play is important on deciding.
I would certainly rather have 2 x 390 than a Fury X