It's not really a matter of being outclassed, Decidueye just... doesn't really have any redeeming qualities. Its stats are mediocre at best, its typing isn't particularly useful, and its movepool is nothing to write home about. It can set up, but its 70 base Speed and medium offenses don't make for a very threatening sweeper. It can provide some support with moves like Knock Off and Tailwind, but it isn't bulky enough to keep these moves coming all game. It can try to do both, but then it's just an awkward hodgepodge of a Pokemon.
thank sumwun for asking this
Decidueye isn't good. It's been NU for every time it was introduced into a game. The Grass / Ghost typing doesn't help it's 75 base Defense stat, being weak to common offensive types (that being Dark, Ice and Ghost). Its movepool is nice, with moves like Knock Off, Tailwind, Defog, Roost and others, but it can't make up for its poor bulk and bad typing. 107 base Attack is fine but won't be going on OHKOing anybody without any boosts. It tries to be a good offensive tank or maybe even a support Pokémon, but it just... can't.
It was good in Gen 8 NU:
Decidueye's fantastic STAB combo and offensive versatility make it a great wallbreaker in NU. Only a handful of Pokemon wall Decidueye's STAB attacks, and even fewer are fast enough that its teammates can't exploit them after U-turn uses or double switches. Furthermore, since Decidueye can use both physical and special sets to great success, finding defensive counterplay isn't so simple until you know its set. Despite these advantages, Decidueye isn't perfect, and its Speed is its most notable flaw. Even though its Shadow Sneak is strong when boosted, faster attackers like Talonflame and Drapion easily revenge kill it, with only the former being susceptible to Decidueye's priority attack. Decidueye also struggles in certain matchups, especially against Dark-types like Guzzlord, Scrafty, and Sneasel, since it's forced to use U-turn more often instead of wallbreak. Lastly, because there are so many great Grass-types in NU, Decidueye can sometimes be hard to justify using over other options.
Also a nice one in Gen 7 NU:
Decidueye is a potent wallbreaker thanks to Nasty Plot letting it break past even checks like Incineroar and Garbodor with Z-Moves. Decidueye's able to use its good defensive typing, recovery in Roost, and bulk to switch into Pokemon like Heliolisk and Slowking to set up and stay healthy. Additionally, Decidueye can provide further utility to teams via U-turn and Defog. Unfortunately, Decidueye's Speed tier is average at best, leaving it outsped and revenge killed by threats like Rotom, Delphox, and Sigilyph. Decidueye is also easily Pursuit trapped by Aerodactyl, Sneasel, and Absol because of this. Furthermore, Decidueye struggles to break past Incineroar without prior chip damage, and its Defog sets are much more passive than its Nasty Plot one.
But the last two aren't Doubles, because I couldn't get any good info about Doubles Decidueye. It was nice in the lower tier Singles as a great wallbreaker, though.
