Polteageist really is a one trick pony. Granted, that trick is terrifying, but Shell Smash Stored Power sweeping is Polteageist's claim to fame, and outside of that, Polteageist is really physically frail, and at an awkward speed both in and out of Trick Room. If you want a powerful offensive ghost, generally Gengar and Chandelure have Polteageist outclassed in immediate speed and power. Poteageist's SDEF is decent, but poor HP makes it difficult to take hits and provides Polteageist very few opportunities to setup, as even the stuff it switches into can dispatch it provided they are not locked into a Normal/Fighting move and not named Sir'Fetched. Even after setup, Poteageist remains vulnerable to non Quick Attack/Mach Punch/Extreme Speed priority, especially Sucker Punch.
Polteageist is not recommended at all for the doubles format. While possessing immunity to Normal and Fighting moves is nice, its extreme fraility makes it vulnerable to any physical spread move (Earthquake, Rockslide, etc.) and doesn't resist any spread moves outside the rare Boomburst and Sludge Wave (namely from Toxtricity). Additionally, boosting is difficult in doubles since that tends to invite focus firing which makes it difficult for Polteageist to survive.
Other Moves
It should be noted in unrestricted formats, Polteageist gains a lot of utility with Baton Pass, as a Smash Passer that can competently sweep with it is a great role and gives Polteageist a lot of value as team support.
Foul Play -Polteageist is expected to be a special sweeper and many people will be targeting its low DEF, so Foul Play is a good surprise move. Shame its coverage is redundant with Shadow Ball
Checks and Counters
Worse case scenario -Polteageist has setup a Shell Smash and is looking to crush your team, what do you do? Send in a dark type. Dark's resist/are immune both of Polteageist's main moves, so as long as they can stomach a boosted Giga Drain, they can dispatch a Shell Smashed Poletageist with ease. Incineroar, Hydreigon and Mandibuzz resist Giga Drain, while Umbreon has phenomenal special bulk to fill this role. Other Dark types can come in, but good prediction will be needed, as a Boosted Giga Drain will still 2HKO most of them (and Bisharp is 2HKOd by Shadow Ball).
As mentioned before, priority is the other key factor. Polteageist has paper thin defense and crumbles under most priority.
252 Atk Choice Band Weavile Ice Shard vs. 0 HP / 4 Def Polteageist: 132-156 (50.5 - 59.7%) -- guaranteed 2HKO
252 Atk Life Orb Mimikyu Shadow Sneak vs. 0 HP / 4 Def Polteageist: 190-226 (72.7 - 86.5%) -- guaranteed 2HKO
252+ Atk Life Orb Barraskewda Aqua Jet vs. 0 HP / 4 Def Polteageist: 129-152 (49.4 - 58.2%) -- 96.5% chance to 2HKO
Please note that you are still required to take a hit, so some bulk is necessary. And some Polteageist may be running Focus Sash, in which case, they will live through one priority attack, but any priority will be bringing them down to their Focus Sash.
Unaware Clefable does have enough natural bulk to deal with Polteageist, but Stored Power still increases in base power, even if Clefable is ignoring the SATK boosts, a 2nd Shell Smash will still hurt a lot (Quagsire is utterly wrecked by Giga Drain and thus can't block Polteageist at all).
+4 252 SpA Polteageist Stored Power (260 BP) vs. 252 HP / 252+ SpD Unaware Clefable: 223-263 (56.5 - 66.7%) -- guaranteed 2HKO after Leftovers recovery
Finally be aware of counterplay in open formats where Baton Pass is allowed, as Polteageist can just Baton Pass its boosts to a counter of your counter.
