Titan's are great alternative to the stupidly expensive Tundra. Titan's biggest flaw is manifolds cracking (all years). Mostly just an irritating noise, but if the crack gets big enough to let fresh air into the exhaust, it messes with the o2 readings and nukes the close coupled cat, which then back-feeds abrasive dust in the engine....which has a predictable outcome. The fix is to go aftermarket long tube headers with a tune. If you're in a emissions state, this is a problem. Some have removed the stock manifolds, drilled and welded the crack with success and reinistalled. But it's not fun.
Other known issues:
-Early year trucks have radiators with intigrated trans coolers which crack. This contaminate the trans with coolant. (Milkshake of death M.O.D) Repair is new trans/radiator. To avoid, pre-emptively update the radiator. Most older trucks this would have already been done.
-Early year trucks had wussy front brakes that warped easily. Later year trucks brakes bolt on easily. Will probably need new brakes anyway....no big deal. Again, probably already addressed by now.
-Exhausts rust apart just after the primary muffler. Exhaust then blows directly on a wiring harness and cooks it. Be aware, and fix it immediatly to avoid significant issues.
- somewhat a point of contention on if this is a real problem or not: Rear diffs and front diffs were "weak" in earlier years. Normal use should be fine. Heavy towing or abusive off road, look for later year truck.
-Rear axles were prone to leaking at ends, soaking parking brake shoes in oil. R+R seals with updated design, and R+R P brake.
-Some early trucks experienced timing chain issues.
Those things addressed, the Nissan 5.6 is awesome, way more responsive than the Toyota. Titan's transmission is solid as well, assuming no M.O.D. Both engine and trans were variants of existing tried and true units.
I own an '13 Armada (based off the titan). Most of the above problems were already sorted in production several years prior but I did "have" to do the headers. (there's a very nice power bump with headers/tune). I'm at 155k now, 4 years into my ownership and still love the truck. With the money saved by no Toyota Tax, even with the exhaust mods, im still literally thousands ahead of where I would have been purchasing the Toyota.
Resale value is obviously lower, so if you buy one, and plan to drive the hell out of it.