Well, Heracross doesn't give a crap about Toxic, especially Scarf Heraboss. ChestoRest Kingdra might also provide some problems for Chansey, since it'll just set up on everything it throws at it. Curse Snorlax with Rest or just Snorlax with Immunity (not sure why anyone would run Immunity, but hey) beats Chansey by just Cursing up in its face, Resting off Toxic, and Body Slamming the blob. Dugtrio traps it, Sigilyph sets up on its face, Aerodactyl can just break through it with the Stallbreaker Taunt + Roost set that's currently on site for Gen IV.The lack of good fighting types in the meta also leads to chansey being really good. Heracross and hitmontop are the only strong ones, and even LO hitmontop can't OHKO chansey with Close Combat.
Personally, I think Chansey is a bit overrated in Gen V UU, but I can certainly understand where all the complaints are coming from. It's one of the, if not the, bulkiest wall in UU currently.