Other Options

Ninjask can opt to use Dual Wingbeat with Heavy-Duty Boots. Doing so greatly relieves the pressure to keep entry hazards off, making Ninjask easier to fit on teams and use. However, doing so sacrifices a lot of power, and Ninjask's sweeping potential and ability to force switches will sharply drop as a result.

Checks and Counters

Stealth Rock Users: Stealth Rock users like Diancie, Mudsdale, and Copperajah realistically won't drop to Acrobatics even with prior damage and easily take the opportunity to set up Stealth Rock, making it hard for Ninjask to come in in the future. They also easily KO Ninjask with STAB moves or coverage.

Offensive Rock-types and Steel-types: Rock-types and Steel-types like Tyrantrum, Aerodactyl, Stakataka, and Escavalier use their STAB moves and coverage to force Ninjask to retreat while taking little from U-turn.

Fire-types: Flame Body and Intimidate users like bulky Talonflame and Arcanine can safely switch into Ninjask's unboosted attacks and proceed to threaten it with OHKOs or status. Furthermore, their slower pivoting options in U-turn and Teleport allow them to grab momentum from Ninjask. Note that Talonflame needs to be wary of a KO from +2 Acrobatics.

Rocky Helmet: Users of Rocky Helmet or Aftermath such as Garbodor and Xatu can tack massive amounts of damage on Ninjask as it attacks, further reducing its ability to come in with Stealth Rock on the field. The extra damage also greatly hampers Ninjask's ability to sweep, as setting up becomes more dangerous and it becomes more susceptible to priority moves.

Priority Moves: Extreme Speed, Sucker Punch, and Shadow Sneak users such as Arcanine, Toxicroak, and Decidueye can give Ninjask quite the headache, as its terrible bulk means it has a hard time enduring such moves. Fake Out and First Impression are ineffective, though, as Ninjask will just block these moves with Protect.