Minior=god tier Gupreet explained why the sets had some issues, but lemme go into a little more detail.
First, Cosmic Power is one of those "good yet not good" moves. Once its going, and if your opponent can't remove it, it's friggin' great. You're this unstoppable behemoth of defense that can only be brought down by a lucky crit. The major problem is it takes a whopping six turns of set-up to get going, which is a really long time of letting your opponent do as they please.
And then there's the issues of spending all that time setting up only to get hit with Haze, Spectral Thief, Clear Smog, Topsy-Turvy, Heart Swap, Dragon Tail, Circle Throw, or Whirlwind. Or the other issue of your opponent seeing Cosmic Power and immediately switching in a Simple Shell Smash user and blasting your maybe +2 butt with a +4 Stored Power Trip or +4 Moongeist Beam, Sunsteel Strike, Photon Geyser. Or +4 V-Create or Thousand Arrows or... you get the idea.
Imposter can also wait for you to set-up, switch in, and just laugh as your unboosted attacks don't even tickle it.
In particular, the Aggron set gets beaten by 99% of the defensive Pokemon in the meta, beating the occasional regular Diancie wall. Most defensive Pokemon run minimal speed, so Gyro Ball's power will be significantly reduced, and they have the bulk and recovery to easily stall out its 8 PP. Aggron's only hope is to Spectral Thief a big attack boost, but that won't work on stuff like set-up Poison Heal or Simple Regigias or a stat-stealing Fur Coat/Unaware Arceus.
Side note: Toxic is used some, especially on lower ladder. Corossive Toxic is particularly effective since Steel-types are rarely prepared for it.
Zygarde is in the samish boat. More consistent damage, but lower maximum power. And harder to PP stall. It also has to scout for Ice Beam, Blizzard, ice Z-moves, and so forth. It'll probably beat most Electric-types at least. Also, Sitrus Berry? That's for Unburden, Harvest, or stray Acrobatics sets.
Giratina... just doesn't do anything except be kinda annoying and waste some time. You might get a forfeit victory from someone impatient, but most players will have one or another way to work around it. This set is also 100% Taunt bait, which shuts it down HARD. It also gets beaten by set-up sweepers who don't care about Normalize, such as Judgement and Revelation Dance users. And you can't haze them because they'll just bop you neutrally because you lose your defensive boosts and can't recover without dropping farther.
Also, Imposter wrecks all these sets and can PP stall them willy-nilly.
For offensive sets...
Lunala's is more or less all right. Might prefer Smash since it's not super speedy and Salac is, as said, a weird choice. It has no options against Dark-types, however. Especially Bounce/Poison Heal users. Also, consider Dawn Necrozma.
Solgaleo... worth repeating: Shift Gear over D.Dance. Set is a little weird though. Psychic Fangs is superior to Zen Headbutt, but Photon Geyser is usually superior to both. Bullet Punch seems a little odd. Potentially useful vs Smash Diancie or something... but otherwise, you're boosting your speed either way. Also, again, consider Dusk Necrozma.
Gurpreet Patel (Sent you a Friend Request)
I kinda agree with Morogrim here, Nagandel's offensive stats are just a little too low. It needs to plug this up with an ability, but other than Simple, Contrary, or maybe Sheer Force, nothing comes right to mind. Some Pokemon can use really strong abilities like Galvanize to make-up for a below-optimal offensive stat, but not sure there's one that suits Nagandel well enough to do that.
I think the place to start with Nagandel, rather than looking at sets, is what meta Pokemon does it check well that others would have a harder time with? Like, in theory, it matches vs Xerneas better than Ray since it's not weak to Fairy and has a SE STAB. It's less scared of Zygarde than Gengar because it can hit with a SE STAB. What sort of team would want Nagandel to answer these threats that it can't do well with something else due to synergistic issues?
Granted, the stats might hold it back from these theories being practical.
