Having owned 3 G's, my biggest concern long term was a combination of availability of some parts AND the cost of parts in general. Even routine maintenance at the dealership is ridiculously expensive, but most work can obviously be done elsewhere. A brake job on an AMG G is CRAZY expensive because the pads and rotors are $$$$. I had pretty good reliability on my G's, but they definitely weren't perfect.
My first G, a 2014 G550, had a window regulator failure (the driver's window went up and down very slowly) which the original dealer claimed was "normal behavior" because the other G they had on the lot had the same issue. A second dealer agreed it was an issue and took care of it under warranty. Otherwise, I don't recall any issues with it in the ~1.5-2 years I owned it.
My second G, a 2016 G550 didn't have any issues that I can recall. I traded the 2014 in for it for the new engine and the optional adjustable suspension, hoping the suspension would help it handle better on the highway. The adjustable suspension was kind of a mixed bag as it made the ride quality kind of harsh in sport mode, but it did handle better at highway speed.
My 3rd, and final G, was a 2017 G63. This one had a few issues in the ~3 years I owned it. Both rear tail lights cracked and the dealer agreed it was most likely because the mounting bolts were over torqued at the factory. It took about a YEAR to get replacement tail lights and the cost if out of warranty would have been about $1500 each, but they were replaced under warranty. I also had an issue with the interior dome light not working. They initially thought it was a defective module, which also took a long time to get. When the module eventually arrived, it didn't fix the problem and they traced it down to a short in the wiring harness, which fortunately hadn't caused a more serious problem. When I traded it in, the Toyota dealer noticed (AFTER giving me a trade in price) some rust bubbles on the passenger side rear quarter window, but honored the trade as they had missed it. The G had been garaged at home, so I was pretty surprised that rust had developed that quickly, but it would have been covered under warranty (if I had noticed it). There were a few little things like the rear door squeaking (solved with some lubrication), but nothing even remotely serious.
So overall, they were pretty reliable, but I decided that keeping a G long term would be somewhat financially crippling due to parts costs. The time it took to get the above mentioned parts also concerned me for the long term. The fact that the parts were ultimately available was good, but what if the part was something that was critical to being able to operate the vehicle (I don't have multiple vehicles)?
Parts availability for the Land Cruiser is something that concerned me as well, especially since I had a Heritage Edition. What if one of my headlights, that are HE specific, needed to be replaced? I'm sure that Toyota made some extra HE specific parts, but most likely just one batch and once they're gone, they're gone. Non-HE parts could obviously be swapped in, but there were a few parts here and there (such as replacement trim panel clips that aren't even LC specific) that I tried to get that were really difficult to find.
