Simply put, the problem is this set:
Breloom (Sneasel) @ Focus Sash
Ability: Technician
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Spore
- Beat Up
- Mach Punch / Poison Jab / Icicle Crash
- Swords Dance
Sneasel's Beat Up lead set forces the use of extremely obscure answers such as Safety Goggles Rhydon x Gallade in order to prevent it from steamrolling entire teams from Team Preview. Sneasel can Spore whatever the opposing lead is with the aid of a Focus Sash, set up SD, and start spamming Beat Up, a move that, when boosted, can achieve feats such as OHKOing Gligar x Gyarados and just about anything that doesn't resist it.
There are two reasons why Beat Up, a usually useless move on other tiers, gets the chance to shine in Cross Evolution. Beat Up's BP depends on the base Attack stats of every member of your team, and the average base Attack stat is higher in Cross Evolution than in standard tiers, thus Beat Up is also more powerful. Sneasel x Breloom also has a very high Attack stat (which is used for damage calcs other than the move's BP), and gets STAB and Technician boost on Beat Up.
Sneasel's Beat Up set has very few checks and most of them can be dealt with the appropiate coverage move. Sneasel x Breloom has 150 base Speed, and with +2 Mach Punch it can outpace and beat most other priority users or, alternatively, if they outspeed or outprioritize Sneasel, survive the hit with a Focus Sash and either retaliate with Beat Up (which is likely to OHKO the target if it doesn't resist it) or sleep it with Spore. With the new Gen 7 mechanics, Sneasel is now immune to Prankster as well.
Perhaps the only non-gimmick Sneasel check is Spritzee x Milotic, but Sneasel can sleep it and even use it as set-up fodder while Spritzee relies on Sleep Talk rolls picking Moonblast (33% chance), and pressure it or even take it out with boosted Poison Jabs. In any case if Sneasel is packing Poison Jab, chances are Spritzee will get severly damaged in the process of taking out Sneasel, which combined with its low Speed means that it cannot perform its role as bulky wincon later in the game.
Another potential check is Safety Goggles Rhydon x Gallade. With Lum Berry you step out of gimmick territory but Sneasel can just then use Spore twice. Earthquake and Close Combat can break Sneasel in two hits taking Sash into account and Sneasel might be discouraged of using Beat Up since Rhydon x Gallade both resists Dark and has Justified as an ability, maxing out its Attack after being hit by Beat Up. Rhydon also resists Poison Jab and can take Mach Punch, but will fall to Sneasel if it's packing the rare Icicle Crash.
Perhaps Sneasel's Beat Up set biggest drawback is that it doesn't work well out of the lead position because of Beat Up's mechanics. However, spotting a Sneasel during Team Preview is no guarantee that it will be the opponent's lead. Sneasel has access to different viable sets such as Hitmonchan or Aurorus that work outside of the lead position. Even if the opponent leads with a Sneasel x Breloom, there's no guarantee that is the Beat Up lead set and not the Spore + 3 attacks one.