TBH the problem is only assist trophies and to a lesser extent pokeballs.
Regular items aren't bad (though without is better)
I'd definitely say some of the normal items have their fair share of problems as well.
The Gust Bellows are basically a free stock against foes trying to recover.
Death's Scythe is an automatic death over 100% with no counterplay.
Bob-Ombs and the Home Run Bat are items with incredible damage and knockback output with very few drawbacks, if any, in using them.
The Super Star literally grants invincibility.
Certain Assist Trophies and Pokémon like Nightmare or Palkia render the game almost unplayable, while most others give you another opponent to keep tabs on. The latter also extends to the Galaga and Beetle items.
Dragoon and Daybreak pieces encourage spawn camping to collect the pieces for low-risk, high-reward KOs.
Beast Ball has little to no counterplay.
The Staff promotes excessive camping due to its range dictating its power.
Ramblin' Evil Mushroom's screwy interface shenanigans make the game unplayable.
Timer can lead to situations with no viable counterplay due to it being Witch Time with more duration and no drawbacks, ensuring your death.
Hammers and Golden Hammers are way too strong.
And of course, certain Final Smashes are way too overpowered even in Meter Mode, which is supposed to be a weaker version. It overcentralizes the game around which characters have a one-hit KO Final Smash, as seen by various Final Smash Meter-enabled tournament events.