Cursed Body's ability to disable a move isn't its main draw. The biggest advantage Gengar gets out of Cursed Body is the fact that it grounding Gengar ensures that it is affected by Terrain. This means it can viably run Thunderbolt on offensive sets if it is paired with Tapu Koko and can avoid Sucker Punch / other priority entirely if it is paired with Tapu Lele, which makes Choice Scarf sets much more dangerous. Misty Terrain is a bit more situational, but at least it protects Gengar from paralysis.
In UU, losing Levitate sucks. Cursed Body confers exactly zero benefits to Gengar in UU due to the absence of Terrain. Most of the Dark-types that threatened Gengar in OU last generation now reside in UU, which leaves Gengar easier to offensively check. Bisharp and Krookodile, which are both very good Pursuit users, greatly threaten Gengar. Gengar's switch-in opportunities are also limited, as it can no longer come in on Choice-locked Ground-types such as Krookodile or defensive Ground-types such as Swampert and Hippowdon for fear of Earthquake. While this is true for it in all tiers, it is exacerbated in UU as the metagame simply is not favourable to it. It is also easily revenge killed; again, this is more of an issue in UU, as unboosted Shadow Balls from Choice Scarf are much more difficult to spam in UU due to the abundance of Dark-types and Pursuit trappers that can remove Gengar. Gengar is still OK, but not excellent, in UU.
Gengar's niche in OU is being a fast and powerful Pokemon with Ghost-type STAB that can either wallbreak, sweep, or stallbreak, depending on its set. There aren't many Ghost resists in OU now that Tyranitar, Bisharp and Weavile are much less common, and Chansey isn't the best way to deal with Gengar as Taunt on Life Orb / Ghostium Z sets does a number on it. Alolan Muk is obviously an issue for Gengar, but Alolan Muk is quite uncommon due to Landorus-T being on every second team. Both variants of Greninja also threaten it, but neither of them can switch into Life Orb-boosted Sludge Waves at all.
Mega Gengar is one of the reasons, along with Ghostium Z Arceus-Ghost and Lunala, that a Ghost resist is mandatory on Ubers teams, whether it be Arceus-Normal or Yveltal on more offensive teams or Arceus-Dark, Porygon2, Alolan Muk, or defensive Yveltal on defensive teams. This generation, it gets its Speed buff on the turn it Mega Evolves, which means that Protect is no longer necessary, freeing up an important moveslot, and that Gengar can check a wider pool of Pokemon, such as Extreme Killer Arceus. It pairs really well with much of the tier, such as:
・ Xerneas, as it appreciates Xerneas's ability to beat Dark-types and can threaten Toxapex, Chansey, passive Steel-types and other Xerneas checks in return, depending on its set;
・ Yveltal, which provides Gengar with a Dark / Ghost resist as well as U-turn, which lets Gengar pivot in safely; and
・ Primal Groudon, which beats Primal Kyogre and Ho-Oh while setting up Stealth Rock and potentially spreading Toxic, which helps Hex sets.
Shadow Tag lets Gengar pick and choose its matchups, coming in on Pokemon it threatens and often guaranteeing some form of damage thanks to it restricting their ability to switch out It can force trades or stallbreaker with the set Chase included in the OP, serve as an all-out offensive Pokemon that can harass passive status users with Substitute and three attacks:
Gengar @ Gengarite
Ability: Cursed Body
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Substitute
- Sludge Wave
- Focus Blast
- Shadow Ball
or wallbreak with a Hex set:
Gengar @ Gengarite
Ability: Cursed Body
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Will-O-Wisp
- Focus Blast
- Hex
- Sludge Wave
Mega Gengar is further customisable depending on your team. It can run Icy Wind, which is extremely annoying to Mega Salamence, Reflect Type, a gimmicky but occasionally effective option that lets it safely escape from Pursuit users, or a Perish Song set that can beat certain passive Pokemon that can't do anything much to Gengar. Naturally, it is not invincible. Levitate being gone is still fairly annoying, as it cannot safely come in on weakened Groudon on the turn it uses Precipice Blades. Gengar also has more problems against defensive teams than it did last generation thanks to Alolan Muk being such a solid check to it, and it does not appreciate the prominence of Arceus-Ground, which handily beats Gengar. Even offensive teams can be a bit problematic, as Gengar does not enjoy how common Choice Scarf Lunala is on this archetype. Nevertheless, it is one of the best Pokemon Ubers has to offer and is very useful on most offensive teams.
Good that u replied late to the thread or else we wouldn't have anything remaining to say after this detailed analysis.