Reveal scenes are lame when it is being revealed only to the Audience.

Obviously this excludes exposition and world building where the audience has to be informed of what the people in the story already know.  But none of that is ever presented as a shocking reveal scene.

There are occasionally "reveals" where it is only the audience learning something we didn't already know and any characters who didn't already know this still don't.  Those reveal scenes are no longer interesting on a re-watch.

In fact a lot of my favorite reveal scenes are ones where the audience already knew the relevant information and it's only characters in the story learning something new.  Like scenes where the Superhero we're following reveals their true identity to someone.

This relates to my advocacy for watching the 6 Star Wars movies in chronological order.  If you think that scene in Empire Strikes Back is somehow pointless if the audience already knows Vader is Anakin then you didn't get what the scene is actually about at all, it's about Luke learning that information and his reaction to it.

This issue is never a deal breaker.  In fact because I like in scenes where the characters learn something for the audience to not be distracted by being equally suprised then revealing it to the audience early to set that up can work out fine.  That's why I tolerate how PLL season 3 handled the Toby working with Mona story-line.  PLL gets away with breaking a lot of my rules actually.

Sometimes a very long running show, I mean shows so drawn out it takes them 30 episodes to cover what PLL covered in 10, will try to spice up the show by revealing something to the audience but no one in the story not even the people most effected by the reveal in question.

In season 1 of Sailor Moon the audience obviously always knew that Mamoru/Darrien was Tuxedo Mask whether the show was treating that as known information or not.  And this is further complicated by how what I'm about to talk about was the case only in the 90s Anime version of the story.  But a certain episode reveals that Mamoru doesn't know he's Tuxedo Mask, it's like a split personality or something.  But this is "revealed" only to the audience.  And the thing is that particular issue is resolved before anyone else ever learned who Tuxedo Mask was, so it kind of never actually went anywhere.

Then Sailor Moon R has a few episodes that are trying not to feel like filler episodes, and yet the only even slight change to the status quo is in how much the audience is supposed to know.

Basically reveals that reveal things only to the audience give the impression of a change to the status quo that really isn't a change at all.  Even if some of these seemed cool the first time, on a re-watch where you the viewer already know everything, they are complete duds.  

Puella Magi Madoka Magica is a show known for it's twists.  However in each case the point where this "dark secret" is revealed to the audience is also when some character in the show first learned it. To me it is those characters learning this information and reacting to it that actually makes those scenes good scenes that hold up even on repeat viewings where I already know everything.

And that's why as I've said before it annoys me that Madoka spin offs are always creating new twists instead of just letting the dark secrets the universe was originally built on continue to drive the drama, I wanna see how different characters would react differently to the same information.  Surprising the audience is not what actually made Madoka a good show.