Regulation M-B introduced two forms of Mega Raichu, and they could not be more different. Mega Raichu Y comes with No Guard — every move it uses and every move used against it hits with 100% accuracy, no exceptions. Mega Raichu X gets Electric Surge, which sets Electric Terrain automatically on switch-in.
Electric Terrain does two things that matter competitively. First, it prevents all grounded Pokémon from falling asleep — Spore, Sleep Powder, and Yawn all fail against any Pokémon touching the ground while the terrain is active. Gholdengo already blocked status moves with Good as Gold; Mega Raichu X now provides terrain-wide sleep immunity for every grounded teammate. Second, Electric Terrain boosts the power of Electric-type moves by 30% for all grounded Pokémon, making Raichu X's own Electric attacks hit significantly harder.
The interaction with sleep is the more significant competitive implication. Spore from Amoonguss and Breloom has been a defining threat in Champions because it takes a Pokémon out of the game entirely. Terrain-based sleep immunity removes that option for the duration of Electric Terrain (5 turns base, extendable with Terrain Extender). Teams with Mega Raichu X effectively force opposing sleep users to wait out the terrain or target the Raichu directly.
Mega Raichu Y's No Guard is a different kind of powerful. Blizzard hits 100% of the time. Stone Edge hits 100% of the time. Any move with reduced accuracy becomes fully reliable. The tradeoff is that opponents also have perfect accuracy against Raichu Y — moves that would normally have a 30% chance to miss no longer have that chance to miss.
Both forms change how opponents have to play. Use the speed tier list to see where Raichu sits relative to key threats.