Should this cost 1 less?

Does it even do anything useful in most environments?

Well, it's a 'Card With No Home' after all. It certainly triggers afterlife a lot in its colors and then a bunch of ETB abilities, so while it may not be for "most environments", it certainly can be attractive for some.

The effect that is. The cost is another issue.

Also:
Leaves battlefield abilities
Destroys all tokens
Resets clones
Destroys all auras
Removes all counters that didn't come with the creature
Regains control of stolen creatures
Does a pile more things when combined with Quicken-like effects

Plenty of useful stuff. It should be a rare environment when it doesn't do much of anything. If there's a real question to ask, it should be "What does this do that a mass Flicker for creatures doesn't do." (And maybe "Why isn't there a clean mass Flicker for creatures yet? Not counting the one-sided Sudden Disappearance of course.") I think the answer "Because." is fine though. I'm unclear as to why mono-black should be allowed to get a mass flicker, though.

I get the impression that this is one of those cards that looks weak, but is much stronger than it looks at first blush. This card pretty much does 80% of what Aether Snap does, and then some (though, admittedly, Stagger Reality doesn't combo with Dark Depths.)

In theory, one could argue that Sudden Disappearance is considerably more expensive, not only because you can potentially abuse the time delay, but because you can target a single player with it. But that's not always true. If your opponent is playing a lot of token creatures, and you have a lot of enter the battlefield abilities, then Stagger Reality will be an absolute wrecking ball.

I wouldn't cost this less than three. Personally, I'd cost this as . The restrictive cost (and the fact that it's unlikely that this will do anything truly destructive early even if you can cast the spell on round 3) should make up for the card's hidden power.

I think this could work either as or

i have no idea why you have a colorless mana symbol in there?? it looks pretty bad (i've used it, ironically.)

Also I think this works in both colors good enough, maybe better in black though. This definitely should be an orzhov card one way or another. Even could be watermarked i feel.

The colourless symbol I chucked in when I realised that this does feel like a "mess with reality" card and aside from blue, colourless does that a lot, Spatial Contortion.

I understand that black shouldn't have a mass flicker but I feel this goes into Lightning helix territory. Can black wrath? Yes, can black mass reanimate? Yes it can. This is like a living end but the creatures that come back are the same as the ones who left. With that in mind, white can wrath and can mass reanimation, although usually with some rider and I feel this has enough of a rider for that. So, yeah it feels like a card that could concievably be in either white or black, the poster child space for hybrid mana.

With that said, the colour identity of "colourless" is murky. As far as I can tell, grey can do anything that any colour can do but not as well as the colour itself. Actual colourless though seems to be able to do anything it wants even better than the colour it's immitating. Kozilek, the Great distortion draws cards and counters spells more effectively than any other blue card. Reality Smasher is just Charging Monstrosaur with upside. Thought-Knot Seer is a Brain Maggot with upside (kinda).

I feel like 3 cmc is the best spot for it since sudden disappearance also hits noncreatures, improving it's strength. It could easily do without the but then you end up with some ugly cost like

i've gotta say, imo is super ugly no matter what you do with it. But this makes sense to me now, i'd definitely advocate this can be hybrid, but honestly is just cool to be white black.... Sadly the means you have to build around it or at least land base well...

anyway pretty interesting when you dig into the color pie around it.

Strict colorless mechanics are in their infancy. Compare Alpha's color pie with today's. Blue got everything because "Wizards are blue" and "A Wizard does it" - in a similar fasion strict colorless cards now get a lot of mechanics assigned because they feel "odd"/"otherworldly"/"messing with reality"/etc. The more the mana symbol gets used the more refined its mechanical representation will become. As a custom card designer this means you get to pick and choose.

If you are interested in my take on what the mechanics would be like (from the Eldrazi perspective) you may check my preliminary notes here.