These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Soul Hackers
Complacent Gaming Syndrome: Sure, you can use all of your demons abilities and have the main character attack. But as soon as Nemissa gets her ma- level spell and you keep putting points into her magic stat, she does so much damage there's not much reason to use anyone else besides upping your demons' loyaltynote which can be done just as easily by using some sake to change their personality and have them just Guard and the occasional enemy who's immune to Nemissa's element. And once she gets the Megido spell? Then all you really have to worry about are a few bosses and the loyalty of your demons.
Evil Is Sexy: Nemissa, depending as much on your definition of evil as on your definition of sexy.
What is arguably her strongest skill set requires that you refer to Hitomi as sexy.
Game Breaker: Once you reach level 59, you can fuse Kartikeya. At first, he seems unremarkable aside from his high speed stat, but if you can fuse him with Tetrakarn and Makarakarn, then you'll find that you have created a demon who can and will protect your entire party from nearly all damage. Because of Kartikeya's speed, he will most often move first, and wise casting of Tetra/Makarakarn depending on the situation, combined with those spells lasting the entire turn instead of for just one attack like in other SMT games, will prevent your team from going down. However, you still need to get his Loyalty up, and of course neither spell works against Almighty spells, but he is still a valuable asset that one would be able to keep for the rest of the game and even into the extra dungeons because of his utility.
Most Annoying Sound: Voice acted menu prompts in the shops. Especially annoying is the "announcement" voice in Paradigm X's forums. The translators opted not to voice minor NPCs, which includes the people inside the forums. But for some reason they left in the announcement voice. It doesn't make any sense for there to be a voice out of nowhere asking which person in a room you want to talk to. But it's there nonetheless. Fortunately there's nothing useful to do in the forums and anyone who wants to hear what the NPCs have to say can turn the voice acting off first.
That One Boss: Mayone. She appears nearly right after you finish the mansion level. She packs a full team of five demons; one of them a Purple Mirror that will discourage you from using spells till you break it, many of them like flinging spells your way, and she has Mudola to cheap-kill the main character and anybody else in any row she targets but his survival is key. Either you equip yourself with anti-Death armor, have a fast-enough demon with Makarakarn to nullify nearly all blows and Mudola, or have a lot of luck. It doesn't help you need to see a cutscene before you're forced to fight her.
Shemyaza. He likes flinging way too many Ao E spells (Mabufula, Mazionga, and Maragion) your way, especially Megidola. His Delta Pulpulae inflicts confusion on all party members, and things will get worse if your demons happen to drop out. If you lose to him, you'll need to watch a three-minute long cutscene + dialogue.
Scrappy Mechanic: The Loyalty system, which requires you to give orders to demons in battle that suit their personalities or risk them becoming less effective in battle, disobeying, or even leaving. It's especially annoying when it comes to demons whose loyalty can only be raised by letting them do whatever they want in battle (in this troper's experience, one such demon had a bad habit of casting a spell that sacrificed itself to restore everyone else, even when there was no need to do so). To be fair, the game does offer the player a few ways to tweak this, either by buying special items to raise loyalty or change demon personalities and by having demons whose loyalty is maxed out follow orders, but it is telling that one NPC you encounter in Amami Airport will tell you that it's a "pain."
That One Level: The Amami Monolith requires an absurd amount of backtracking compared to other dungeons across a huge swath of space, all with a high encounter rate with powerful demons with annoying resistances, including one that can de-level the party. The final dungeon is a cakewalk in comparison.