Breloom

Breloom has a lot of positive aspects, including its movepool, ability, and typing. For one, it has Spore, the only 100% accurate sleep move in the game, which can cripple anything that isn't a Grass-type. This allows Breloom to put pressure on at least one of its checks or counters, making playing around this Pokemon a pain. Also, many of Breloom's other moves benefit greatly from Technician including Rock Tomb, Bullet Seed, and Mach Punch. It can even put slower threats to sleep and capitalize on the free turns with Swords Dance, sweeping weakened teams with Mach Punch. Along with this, Breloom has just the offensive typing it needs to check common offensive threats Colossoil and Kyurem-B.

However, Breloom's horrible defensive typing and stats mean it really struggles against anything faster than it that resists Mach Punch, such as Talonflame, Cawmodore, and Latios, among many, many others. Its typing particularly suffers against Grass-types that can absorb every move coming at them including Necturna and Mega Venusaur. This, on top of its very subpar Speed tier, means that a lot of common Pokemon including Mega Charizard X, Mega Charizard Y, Tornadus-T, and Latios can outspeed and KO it easily, making Breloom overly reliant on Mach Punch.