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.
Broken Base: The fans on the forums after various stumbles done by WeMade, up to and including no notice of the fact WeMade was being bought/buying JoyMax, a rival company.
Made worse when people did not notice there would be a character wipe after "pre" CBT, and later after CBT. Also, the fact it seems to take three or more weeks to stomp bugs is aggravating the situation.
Recently, GDMO stumbled big time with the recommendation button compensation fiasco. Long story short, the "friend recommendation" button disappeared during the Alphamon event, something very inconvenient since one part of the event required this. Joymax made a generic apology and promised compensation. The day of the compensation came, and nothing happened... then it was announced that the compensation was delayed. The new day arrived again, and the promised compensation still didn't arrive, having been delayed yet again, and Joymax promised all players would get something for their troubles. Come the actual day of compensation, Joymax announces that only a very select few players get the compensation (despite their earlier statement making it clear all players would receive it), and despite this the majority of that fraction of players who would have gotten compensation still didn't get it anyway. Understandably, the fanbase is pissed due to the repeated delays, broken promises, and total lack of delivery and professionalism. Joymax has yet to clarify any reason for all the problems.
Game Breaker: Lucemon Satan Mode, if you were lucky enough to get it, can very easily be this, as the egg and Burst Mode item were only available for a limited time. Despite being a Burst Mode, it has stats on par with a Jogress/DNA-level Digimon, meaning once you get it to Level 70 (the level required to use the Burst Mode item), you can tear through armies of enemies. Especially if they're Data or Thunder, and double especially if they're both.
In the Korean servers, Apocalymon is also this, although to a lesser extent than Lucemon Satan Mode, and it applies most to the Soulmon line's version. Being an Unknown type Digimon, Apocalymon can't take full advantage of the attribute system, but it doesn't need to when it too has stats on par with a Jogress/DNA. What's more, in the Korean version at least, the Burst Mode item for Apocalymon, while expensive, is unlimited, meaning you can save up the money for it, unlike with Lucemon's.
Scrappy Mechanic: Dear lord, the egg system. You have a very small chance of getting a mercenary egg from a given Digimon (assuming they're a member of an available family). This alone could take hours, but then factor in the data-insertion system. You need a certain number of data, again acquired through slightly more common drops, and need to apply it to the egg at least three times to get a new Digimon. The amount of data varies from species to species, but the most popular or desirable Digimon tend to take the most data. The problem is that this insertion process can randomly fail and waste your data. On top of that, there's a completely random chance that failing might cause the egg to break. So you might break it on the first attempt, or fail but not break the egg after twenty times. The system is heavily weighted against allowing players to acquire new Digimon, and many have demanded at least a slight increase in the odds so as to not waste so much time and effort. Ultimately, many players find it easier just to buy an egg from the cash shop.
Relatedly, the drop system. After reaching a certain level above them, Digimon have a hugely reduced rate of dropping items. Have fun getting a Dokunemon egg or dragon-species data after a certain point, since it'll take hours, if not days. While this system might be in place to abuse getting too much material too easily, players still cry foul since the Korean version does not have this cap implemented at all, so players of the global version expect fair treatment in that regard.