I think the older Nissan 4x4's had a mildly bad reputation (I'm obviously talking about the North American market). Some issues with transmissions, front axles on the IFS trucks as well as an issue where radiator fluid mixed in with transmission fluid (SMOD).
But to their credit, Nissan corrected a lot of those issues as they popped up. The last version of the Xterra (before they discontinued it) was pretty decent IMO. The recent Titan and Titan XD, while nothing extraordinary, certainly are a step up for Nissan in terms of build quality and capability.
If all these improvements are indicative of what the next Frontier will be like, I'd definitely consider one, especially if it has an optional 2.8l Cummins (there are rumors of Nissan testing a prototype diesel Frontier).