>>13050686That's the thing. Lando has a very good array of coverage moves that hit extremely hard. If they've taken some prior damage, even the best checks can get wasted by a strong coverage move. Rotom has its health bar snapped in half by Sand Force Stone Edge, or sheer force Focus Blast / Psychic. Celebi is turned into mixed vegetables by U-turn. Skarm is the closest thing to a counter, but can't really hurt it back, and Lando can set up on it or just gradually beat it to death with coverage.
There are other things that probably needed to be banned first (Terrakion, Kyurem-B) but this ban still makes a good deal of sense. It's a disproportionately powerful threat.
Fun fact. Now every genie has at least one of its formes banned from OU.
