I don't think Chansey is broken at all. Lets look at the top tier mons, what does Chansey actually beat?
Doublade - switches into Chansey and uses it as set up bait
Croconaw - DD doesn't OHKO, but Chansey can only paralyze it. 252 Special Attack doesn't even OHKO with Thunderbolt. SD can 1 shot Chansey.
Fletchinder - Chansey lives a +2 Acrobatics, but again, Chansey doesn't even come close to OHKOing back, so it can only paralyze / toxic Fletch. Fletch can also set up on Chansey, altho it's a risky plan.
Fraxure - Easily gets past Chansey, even with Choice Scarf.
Golbat - just Taunts Chansey and kills it. Chansey is also free Defog bait. Only has to be weary of Thunder Wave on the switch.
Gurdurr - switches into Chansey and uses it as set up bait.
Monferno - Life Orb CC 1 shots while non LO comes close to 1 shotting.
Rhydon - 2HKO's with Earthquake.
Zweilous - Easily gets past Chansey.
Krokorok - Can get through Chansey with Taunt, Chansey won't appreciate a Knock Off, Chansey beats Scarf though.
Piloswine - SR Piloswine gets walled, but LO Piloswine 2HKO's.
But what about the special attackers? Chansey sure must wall everything with that massive Special Defense of its. Except that's not really the case.
Roselia - Chansey is free Spike bait, and that's what Roselia wants to do with its life anyways.
Electabuzz - Still Volt Switches out of Chansey, and can 2HKO with Life Orb Cross Chop.
Magmar - 2HKO's with Life Orb Cross Chop easily, although Chansey is a pretty good check to this because Magmar has to predict that.
Haunter - Immune to Seismic Toss, can Taunt Chansey and slowly break it down.
Frogadier - Gets walled pretty hard, but it can U-turn into something that breaks Chansey.
Duosion - CM's up and then kills Chansey with Psyshock.
That said, Chansey actually does counter Porygon2 pretty hard. I think offensive tank Porygon2 (or even Agility) is really underrated, but Chansey beats that easily. It also beats Sliggo really well, unless it's Curse Sliggo although idk how good that actually is.
Chansey has Wish and Heal Bell which are amazing, but idk if you can call that broken. It 3HKO's even frail offensive Pokemon at best, and a lot of good Pokemon it only 4HKO's with Seismic Toss. I feel like Porygon2 will compete with Chansey a lot because it's bulkier on the physical side and has better offensive presence. Porygon2's Special Defense is generally good enough anyways.
So yeah, I highly doubt Chansey is broken, or even a top tier Pokemon. It doesn't actually wall as much as it looks like it walls, especially when there's any hazards whatsoever on the field, and while Wish and Heal Bell are amazing, they're not as useful in such a fast paced tier as Middle Cup. Chansey is also a little passive for Middle Cup.