The X6 is a slightly older design - it was developed before we invented our resistive multitouch keyboard technology which is featured in the X4. Like most gaming keyboards (apparently including the Razer Ultimate), the X6 relies on rearranging the key matrix to move the non-working key combinations to places where they do the least harm. So the most important 3-key combinations should work. However, there are still lots of 3-key combinations that don't. The Sidewinder X4 completely eliminates this problem. All key combinations work, up to an absurd number. Before the X4, this feature had only been available in very high-end keyboards.
People frequently ask me if this matters. Most people seem happy with their standard gaming keyboards that have the issue. For a standard matrix, something less than 5% of the 3-key combinations have problems. (Which combinations depends on the specific matrix layout, so it varies from keyboard to keyboard.) So you might get lucky, and never hit one of the non-working sets. However, in my experience, when you sit down with serious gamers and actually show them which sets don't work on their favorite keyboard, they get very upset. A typical reaction is "that's why I can't throw a grenade while moving north-west!" or something similar. Most people tend to blame the game, themselves, etc. when they are having a problem like this. Usually, they don't think to blame their keyboard. Given these experiences, I think it matters much more than most people recognize.
For what it's worth, we've had quite a few requests to license the tech in X4. Microsoft recently kicked off the licensing program. I successfully argued that fixing this problem would help the entire Windows gaming ecosystem, and as such, we should make the cost of the license very low. We've done that. So I am hopeful that you will be seeing this spread widely across the industry in the coming years...
Disclaimer: Yes, I work for Microsoft and I am one of the researchers behind the tech in X4. No, I'm not speaking for Microsoft - these are my personal views.