... Cramorant-Gorging is inherently unpleasant to play against, because it makes one really scared of clicking any moves, such as anchor shot or u-turn because the draw backs are so huge. Not only that, scarf Pokemon are pretty terrible right now because they don't do enough damage to kill Cram, and are totally crippled by it. Typical speed control measures, such as LO Pheramosa have also lost almost all viability because of Cramorant, and the convential way of playing balanced hackmons has drastically changed because of Cramorant. Pivot heavy teams have difficulty finding viability so the best teams have begun to resort to trapping, which coincidentally enough, has occurred simultaneously with the increase in popularity of imprisonform trapping. This has created a meta, which to many is extremely unpleasant to play and watch.
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Cramorant-Gorging's ability to paralyze and chip foes upon switching into any damaging move is unprecedented. It has provided tremendous team support with this, acting as Speed control, entry hazard control, a disruptor with Trick, and a pivot. The threat of activating Gulp Missile keeps foes from choosing damaging moves as they fear being paralyzed and chipped in return. This leads to 50/50 situations where the Cramorant user usually has the upper hand, as they can switch to Cramorant to take the hit and pivot out or make a play based on the opponent's fear.
While counterplay to Cramorant is available and its 70 / 55 / 95 bulk leaves much to be desired, its impact on the metagame has largely been unhealthy and uncompetitive. The paralysis support from Gulp Missile not only has the immediate benefit of keeping attackers at bay, but a long term benefit of forcing its foes to deal with potential full paralysis keeping them from moving. While this is true of paralysis with or without Cramorant, its ability to spread paralysis without even choosing a move separates it and makes it unhealthy.
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So tl;dr it was because of its signature hard-coded ability, Gulp Missile, because of which it was banned. Cramorant in itself isn't good, its ability is something which makes it broken even for BH. Because Gulp Missile is hard-coded only for Cramorant, no other Pokémon can get it (even with hacking), so Cramorant was banned and not Gulp Missile. It also provides lots of utility to balance builds in the form of hazard removal, pivot and a Pokémon which can chip opposing foes easily. It was extremely unhealthy for the metagame and thus was banned.
Hope it helped.
