Reported issues on first gen:
Very first ones, the pin than the throttle pivots on brakes and gets sucked into the engine.
The reports of this were at around the 100000km or below from my research.
The diesel have had ECU software upgrades to get rid of a flat spot in top gear (software, so not a worry about reliability)
In 2004 they went from a 4 speed auto to a 5 speed auto. In 2007 they introduced the diesel, a 3.3 petrol V6 and a 3.8 Petrol V6. The 3.3 is chain driven so no timing belt to change (big plus in my books) the 3.5 is belt driven. 2007 onworads models gain esc, but loose the LSD.
Generally very good cars (i done my research early this year before buying). I have only had mine about 9 months, and no huge issues except for the 4wd switch playing up.
When test driving make sure you go in some sand and test the 4wd as if there is an issue it will be generally be expensive if you dont DIY. But that goes for any 4wd.
One bad point (in my books) is the standard suspension. Until 2007 very sloppy. And the standard height is pretty low. So depends what where you plan to take it. Now i know on paper its as high as almost all of the rest of them as its measured from the lowest point which is the rear diff. BUT the fuel tank and a lot of other things are not that much higher. If you look under other 4wd's everything else is tucked up a fair bit higher than the diff. So if you plan to take it on soft beach sand in my experience you will need to lift it. On paper same 'low point' as the big names, but the rest hangs very low too. Something i didn't even think about until i realised what was happening on the beach. I was getting bogged non stop as the fuel tank was dragging in the really soft stuff and coming to stop resting on its belly. On the harder sand, it was fine. Lifted and new tires and it goes really good even in the soft stuff.
So just think about where you plan to take it and consider the cost of suspension when you think about this car.
The early V6 (3.5) is a strong, stoutly built engine but carries over a few issues from its (20 year old) Mitsubishi origins.
It is a timing belt engine so 100,000 Km belt changes, are needed, it is also an interference engine, so will be a dead engine if you neglect this.
The crank pulley bolt has a propensity to either back out or break off if under or over tensioned. It must be renewed if removed because it is a TTY bolt.
The tendancy of the screws holding the butterfly blades in the variable intake to back out and get sucked into the engine (as mentioned by Bob above) some units were replaced under warranty, or you can just disconnect it.
The crank angle sensor is a bit fragile, often needing replacing after 100,000 Km or so and it is a bugger to get to as it is under the timing belt.
The rocker cover seals tend to leak after 100,000 Km, particularly the (RHD) passenger one. Replacement is the only fix.
Due to an iron block, it is heavy, and somewhat thirsty.
The foam plug in the charcoal canister mounted under the chassis can disintergrate and let the charcoal granules loose into the pipe or even the tank where they may block up the fuel pump intake.
The 3.5 oil system is filled with many fine passageways, so using the correct grade of oil, and changing it regularly is essential or you will get noisy lifters and gunk.
However Hyundai / Kia have reworked the compression ratio, valve timing, and injection mapping to produce a smooth, tractible, strong running engine that unlike the original Mitsu engine pulls well from low revs and runs quite happily on regular or even E10 fuel. This engine is exactly the same as the one fitted to the Hyundai Terracan, and so is the gearbox, transfer case combination.
The 3.3 and 3.8 V6's are modern design, efficient engines, with high power output and relatively good fuel economy for what they are. They don't seem to have any particular vices noted yet, although the hydraulic tensioner on the timing chain can sometimes take a bit of time to take up the slack on a cold start leading to a few seconds of chain rattle.
The diesel is a modern Euro design CRDI engine that delivers good power and torque and excellent fuel economy. A software fix fixed a flat spot in the power curve, a few engines were recalled for over torqued injector retainer bolts which could snap and allow the engine to fire out an injector. Like all CRDI diesels, fuel cleanliness is essential for a long life of the injector system.
The 4 speed auto is a Japanese built Aisin unit, similar to the ones used in Toyota Landcruisers in the 1990's and early 2000's. Aisin autos are reliable and durable. Looked after, they will probably last the life of the vehicle, and they use cheap and easy to find everywhere, DX III oil.
The five speed auto is a Jatco unit built under licence by Hyundai, it is essentially the same as Jatco units used in Nissan Pathfinders and other similar Nissan models. It is also a good unit but requires a special auto oil which is apparently distilled from ambergris or something similar judging by the price.
Some 5 speed autos suffered from irregular shift issues which was caused by electrical interference in a poorly shielded wire in the harness, this should have been fixed under warranty by now.
The part time transfer case is a chain drive, planetary gear, Borg Warner electronic unit as used in many brands of 4WD vehicles. The main issues with them are the electric shift motors and associated wiring. The actual guts are pretty reliable.
The TOD transfer case is also by Borg Warner, they are good for icy roads or snow but watch out for unequal tyres sizes as they can fool the system into thinking the tyres are slipping and this will activate the system when it is not needed, eventually burning them out. The shift motors and associated wiring and ECU can also fail.
The diffs are Eaton which is a well respected brand although a bad batch of bearings caused early rear diff failure in some facelift models. Most should have been fixed under warranty by now.
The front diff in part time 4WD models has a pneumatically powered dog clutch to provide freewheeling when 4WD is not required. This is activated by a small air pump located under the front edge of the battery.
Failure to engage 4WD can be caused by the pump failing, the actuator in the diff failing, or the connecting tube splitting or blocking up.
The suspension (in stock Australian setup anyway) seems to combine too soft springs with too hard shock absorbers, which results in great handling on the tar, but a "jiggly" ride on rough surfaces, and a tendancy to skip about and bottom out off road. It is also a bit short on travel. Aftermarket springs and shocks can fix this and give a much needed mild lift.
My unit (a 2003 V6) was bought secondhand in 2010 with 120,00 Km, it has excellent fit and finish on the bodywork, no rattles, no leaks, and no rust anywhere. The quality of the interior switchgear and carpet is a bit plain but quite servicable. The trim quality is more like a basic Japanese car than upmarket but this doesn't really worry me.
My car required a crank sensor replaced not long after we bought it, I also did the TB at the same time as it seemed that the cheap skate dealer had not replace it at the previous service like it should have been. (didn't look like they did the oil either) I also had to do the rear disk pads and a leaky front drive shaft seal (so much for roadworthy checks) And yes my rocker cover seals weep slightly. However apart from these niggles which are not really more than routine maintainence anyway, the car is is comfortable to drive, is driven every day at highway speeds, tows a camper with ease, and has been totally reliable.
I am quite happy with my purchase. These cars are a bargain compared to a similar specced Toyota Prado, or Nissan Pathfinder and parts a quite cheap, albeit a bit harder to find.