There are some things to look out for, my experience is from my 2007.
Gas Mileage: Abysmal. Typical with this kind of weight and power. In mine I hover around 12 MPG but as low as 8 on the winter blend with remote start warm-ups.
Exhaust Manifolds: Known to develop cracks around 80k - 100k miles. ~$1500 - 2000 installed (prices very greatly but Cajun B-Pipes and JBA headers is what most people get (OEMs will just crack again). The install is a real PITA and best outsourced.
Brakes: Due to the large size, the Armada eats through brakes pretty quickly. Not a bad job, few hundred for pads and rotors. I've tried a bunch of different brands and now just went with the Autozone duralast gold which have free lifetime replacement.
Front shocks: OEMs are terrible - mine were dangerous at like 35k. A lot of people get the Bilstein 4600s. Around $250 a pair, can be installed with hand tools and air compressor and rented spring compressors. Huge improvement in ride and handling.
Rear Air Shocks/Compressor: If this has the auto leveling, many people have had issues with the components of the system. Aftermarket compressor is available (~$160?? from Suncore). OEM air shocks are about $250/pair. Other sensors involved in the system as well. Some have converted to regular gas shocks with stronger springs and spring mounted air bags for leveling when they tow.
Front Anti Sway Bar Bushings: Can dry out/crack - I replaced mine.
Pinion Seals (i.e. Transfer Case): Mine developed a leak, PITA job so had a mechanic do it.
Radiator: OEM is plastic and cracks. The transmission lines run through it and there is a danger the fluids can mix. Many replace with all aluminum radiator (I used CSF 3328 - $350.). Job is only hard because of the limited working space.
Power Steering High Pressure Hose: Leaks/seeps. About $100, not a bad job except the limited space makes it more difficult than it has to be.
Clock Spring: Seems a lot of people have this issue. I don't know much about it but I'm sure its a matter of time.
Driveshaft U-Joints: Changed mine out - not a bad job. Option to get greaseable fittings but you have to be disciplined to grease them regularly.
AC condensate/drainage line: Factory design allows water to run back into the cabin. There is a fix you can do by routing a hose all the way to the bottom of the truck. This one was cheap but a REAL PITA due to the limited working space on the Armada (Titans, where I saw the fix, seems to have more room. I think ours is harder due to rear AC lines. Symptom of this problem is a wet passenger footwell (under the floor mat).
AC Lines (Rear Leak): Several people had issues where the rear refrigerant line developed a pinhole leak. Book says you have to lift the truck off the frame but others here have found a fix which seems to work and is less invasive.
AC Not Cooling while not moving: I went through tons of steps to troubleshoot, but it ended up being that the refrigerant was not charged to factory specs (with both front and rear AC on at the same time).
Battery Drain: Lots of threads on this but basically there is some sensor on the negative battery cable which does not allow the alternator to fully charge the battery.
Edit, a few more:
RUST!: In salt belt states, the undercarriage will rust badly.
Alignment: Front does not have camber (?) bolts, but can be added. Rear's are almost always seized and are a pain to get out. Most people find the bushings have seized to the bolts and end up swapping out the control arms in the back and other suspension components.
Well that's all I can think of for now. I believe the 2004 - 2015 Armadas are the same model so I would suspect these are applicable to a 2014.
The good news is most of the bugs are known but the engine and transmission are supposed to be very well built. I admit I seem to have repaired a lot on my 07 but I sill love it (currently working on the rear shocks/air lines).
