In my opinion about Darkrai I think it's more about people having a sorta "Uber prejudice". It happens all the time something was top once gets dropped, why I remember back in gen 7 people were calling Mega-Mawile and Greninja broken mons who should be booted back to Ubers and both were solid picks for OU, same thing in Gen 8 with Blaziken were everyone was saying it was gonna tear the tier apart... and then it ended up as a mostly mid pick with better fire mons in Volcarona, Volcanion and Heatran. Like there's nothing wrong in being careful about nominations, take Genesect, it always starts as OU and 2 hours later it gets the ban, but we should not be afraid to look at a mon who got hit with some hard nerfs and bring it to OU, I mean Aegis got the boot two times and Zama-H also took some heavy nerfs and while some people still claim the latter it's broken I think most people agree they are fine.
TL/DR: It's fine whenever or not you thought/think a mon is broken, we can always test it, just be polite to your fellow users.