The “2021” QX80 is mechanically based on the 2010 model year, in 2018 it was facelifted but the engine, transmission and other major components remained the same.

And that is a good thing as Nissan’s 5.6L V8 is a big well built proven engine - it can last even 200,000 miles without major repairs.

(The only common repair I can think of on this engine is the timing chain getting stretched by aggressive drivers but that is repairable and a common issue on modern cars)

And the transmission, is also an extremely proven design. It’s a big old conventional 7 speed automatic - these also tend to last for a very long time. (Remember to change the fluid every 30k miles)

This car was designed before Nissan (owners of Infiniti) joined Renault - it is an old school Infiniti like the G37.
It may encounter quite a few issues after 100k or 150k but the car is imo still overall great and reliability should be better then average.

it is more reliable then Hyundai, Kia, VW and almost everyone else but

that being said, the Toyota Highlander or LandCruiser (not on a Diesel engine, only big gasoline engines last on these) are probably more reliable and maybe a better value. (These usually do not have random issues - these are true “tanks”)

Literally no way to know as they are brand new. 

Infiniti doesn't have the best track record for reliability, especially past 100k miles.

I wouldn't buy brand new, but used is not a bad idea. They are pretty dependable (tbh it's really just a more upscale Armada). It has a reliable powertrain with that V8 and 7-speed auto that they have been using for over a decade, so it can last if you take care of it. However, if u are looking for good gas mileage, I wouldn't buy one since they get horrible fuel economy. Other than that, they can be pretty good full-size SUVs.

My family and friends rented a 2016 QX80 when we went to Las Vegas for a roadtrip and we loved it.

I would rather have a Toyota or Honda than a Nissan product since you basically pay the same money for brand new ones. 

I agree with Dan and infinitifxfan’s assessments, and won’t rehash what they said.  But realize this generation of QX80 (which is 2nd Gen) dates back to 2010, so it’s not new.  The engine and transmission mentioned were the only ones offered and after 10+ years appear true and tried (also built on the same platform as the Nissan Titan, Nissan Armada, and Nissan Patrol which are solid choices as well.  Some might call it long overdue for an all-new generation but I say that’s OK:  it’s taking a page out of Toyota’s playbook.  That being said, you are better off looking at a used one and save some money since the powertrain and overall platform hasn’t changed.

Not the best, but not the worst either. Probably middle of the pack for that segment. No way in hell I would pay that much for a Nissan product though personally. You’re not really getting much in exchange for sacrificing the reliability you’d get from something else. In general you’re paying more for a large body on frame SUV like this too vs a unibody 3 row before even thinking about fuel economy. I would be absolutely sure you need something that large before even considering it.