The two main issues regarding 7990 adoption are, according to WCCFTech, micro-stuttering with Crossfire setups and the better dollar value of paired 7970s. The 7990 comes in at the thousand dollar ($1000 USD) price point despite being, for all intents and purposes, nearly identical to a pair of separate 7970 cards. While the 7990 has access to a superior "Never Settle" bundle when compared to twin 7970s, with the addition of a Deux Ex: Human Revolution license, it would be difficult to consider that as an excuse for the $200-$300 USD price gap.
Cost aside, this would be a really odd time to cancel the 7990. The product was launched just a few months ago, despite similar price concerns, only to be allegedly killed right before the driver that makes it worth its weight? If true, either AMD decided to annul their "mistake", or we will get some interesting news when the frame pacing driver finally gets released.
http://www.pcper.com/news/General-Tech/AMD-may-discontinue-Radeon-HD-7990