Let's relax a bit about Chansey guys - sure, it's VERY potent defensively, but that doesn't mean it's broken. Toxic Spikes make it absolutely useless, Fighting-types can switch into it with impunity, while even the special attackers it walls can instead beat it by using Sub + a stat boosting move, Chestorest + a stat boosting move, or Taunt + recovery. With a little prediction you can also Trick it a Choice item. And these are the options offense has, since Chansey is absolutely horrible against stall (set-up fodder for hazards, can't do anything to defensive pokes but being phazed away).
You honestly can't expect to OHKO Chansey. You can use it as set-up fodder though.
-Toxic spikes beats every wall unless they aren't affected by them so thats invalid
-Fighting types have to worry about taking a thunderwave or toxic when switching in..as Flareblitz stated, his choice scarf heracross couldnt kill a chansey and it ended up paralyzing him, and thats one of the strongest fighting types we have in UU.
-a sub+stat boosting move does work..but its not hard to see coming really..and chansey doesnt work alone..mismagius is considered one of the best switchins to chansey(as long as it avoids status move) as it usually runs sub+stat boost, but most ppl would switch to their mismagius counter to beat this..with team preview its easy to see this coming as well
-Tricking this gen is way harder than it was last gen...Rarely will you see a chansey get tricked because trick users are obvious
-taunt+ recover is a rare combo as well, (mew, deoxys)not many pokes have this combo or at least bulky to use it..threatening chansey but it is easy to switch chansey right out..that combo not only works on chansey, but on alot of other pokes
-against stall, I can agree it is a set-up fodder for many entry hazrad pokes or crocune(and identical sets for other pokes)
Also for the majority of those criteria they all have to worry about switching into chansey's status moves
EDIT: I wouldnt like to propose as chansey being god or unbelievebly broken as she does have her weaknesses, but through sheer force it is much easier for chansey to wall the majority of pokes and easier to build a team around her this time around...yeah through great prediction she may lose, but she takes away most of that prediction now