MeowCanuck: The only issue I have with NG+ is with artificial QOL debuffs you unlocked in your previous playthrough with no option to opt out. For example, the differences between FF4 3D and TMSFE. Your map completion % is carried over between playthroughs, which is an extremely helpful QOL improvement for completing Namingway's quest for the Treasure Hunter augment. However, starting a new Lunatic playthrough with TMSFE, your map completion is reset to 0% and it just becomes a hassle to redo everything again especially if I got 100% the first time. I was completely discouraged and haven't touched it since.
In FF4 3D:
* There's an artificial limitation of 3 playthroughs, which is annoying, especially if you assigned augments sub-optimally on your first playthrough (and there's no way to re-assign them).
* You get a reward for clearing a map 100%. If you get this reward on one playthrough, you can't get it on New Game +, and the rewards are consumables which (for the most part) don't carry over. (At least if you give Cecil the Bless augment, you can use it to eliminate the need for Ethers in the mid-game; add one Soma Drop on Tellah and you can spam Meteor in what would otherwise be the most annoying part of the game.)
Also, I am reminded of the SaGa 3 remake. New Game + gives you the option of keeping everything; saying "No" results in you not even keeping jukebox songs, and 2 of the songs will be forever unavailable on that file. (Also, New Game + is the only way to play Hard mode, which really should have been unlocked from the start, as that's closer to the difficulty I'd expect from a SaGa game, while still being easier than the harder entries in the series. New Game + without keeping everything is the only way to get a fresh start on the harder difficulty, which feels like how the game was meant to be played.)