As far as I know, Puzzle & Dragons (PAD) has not nerfed any monsters after they were released. Granblue Fantasy (GBF) is the opposite as Cygames frequently hits newly released characters or mechanics with the nerf bat.
PAD doesn’t nerf
For PAD, people complained about the announced leader skill change to uevo Skuld back in March this year–from 3~3.3x ATK to water dependent on HP, to flat 3x ATK to water and 1.5x ATK for matching exactly 5 orbs with at least one enhanced orb–which could be seen as a sidegrade or even downgrade instead of a buff. However, when it was actually implemented, the new leader skill was a straight buff (3.5x ATK to water, 1.5x ATK for matching exactly 5 orbs with at least one enhanced orb). In general, leader skills are never nerfed in PAD, only buffed.
When Juggler from the Final Fantasy Crystal Defenders collab was first introduced, it was extremely powerful, a 7x ATK mono attribute leader. Back then, few monsters had 6x ATK leader skills, and almost all of them save Anubis were rainbow leaders (this generally made them more difficult to play, and lowered their damage output). Simply put, Juggler was near broken. There was quite a lot of backlash to Juggler, and I recall the community on the PAD forums was ambivalent about it. Despite being pressured to do something, GungHo stood by their decision and did not nerf Juggler. If there were ever a time when they should have nerfed something, that was it.
The modified heart cross mechanic has proven to be extremely strong, and Kaede has catapulted into universal agreement as the strongest solo leader in the game thanks to it; with Gremory, Sumire, Miru, and other heart cross leaders not far behind. In the comments section on one of Setsu’s posts, a commentator speculated that the new heart cross mechanic was a bug, but GungHo could not fix/change it because that would constitute a nerf, and somehow that’s not allowed. I wouldn’t know whether they’re allowed to nerf things or not, but it’s pretty clear at this point that GungHo just doesn’t nerf new characters, even if they’re incredibly powerful and threaten the balance of the game.
GBF, on the other hand
In sharp contrast, GBF sees nerfs to new characters or mechanics quite regularly. Off the top of my head, the characters Hallesena and Korwa were nerfed shortly after release (Hallesena was nerfed in 3 hours, Korwa in 15 hours), Tear Grants from the Tales 2 collab was nerfed within hours, the warlock class (row 4 version of the wizard class) got struck with big nerfs to the nuke playstyle within a day, and the journey drops item drop rate buff was fixed/nerfed after it was realized that it boosted the drop rates of certain super rare drops from 0.1% to 2.1%. This has all happened in the last four months or so, and I’m certain other instances in the past exist.
Time and time again, GBF is not afraid to nerf new content, although to their credit they usually do it within a day. It does, however, instill a bit of wariness whenever something new comes out because anything that seems too good is liable to be struck down by the nerf hammer.
I like how GungHo sticks to its guns, even though leaving Juggler untouched wasn’t necessarily a good thing–it massively accelerated the powercreep curve in PAD, and I think the ripples are still felt today. I feel Cygames has an itchy trigger finger, but at the same time GungHo is just better at designing new stuff that isn’t super broken, thus necessitating quick nerfs. Funnily enough, Hallesena recently got buffed and now she’s quite respectable again. Better to start low and buff later than to oscillate with nerfs and buffs, in my opinion. At any rate, PAD wins this round.