Chansey does not wall every special attacker in UU. NP Mismagius destroys it entirely. Crocune and Sigilyph beat it eventually. NP Togekiss can also destroy it even without Roost. Boorego used Heal Bell with Nasty Plot, and it's more than enough to wreck Chansey since it won't be getting in enough Seismic Tosses, as Togekiss keeps flinching you, and gets more boosts if you try to use a recovery move. Id didn't use Chansey against that Togekiss, but it's not hard to see why it would lose. Zapdos or another electric type is a much better answer to it. You cannot reasonably deal with those four special attackers by using two other special walls alongside Chansey. Chansey does wall what it walls very well, but if using it leaves you open to other special attackers, I wouldn't consider using it.
NP Mismagius dosen't like a possible Toxic or Thunder Wave, the former limiting it's sweeping [Especially as it has to get to +6 in the first place], and the latter making it rediculously easy to be killed by anything that isn't Chansey.
NP Togekiss is a similar case, Thunder Wave makes it lose to Chansey outright [It can't Flinch it to death anymore] and Toxic is a similar case, due to it either needing to set up, or needing time to flinch it to death. 3 S-Tosses will also finish NP Togekiss without Roost, if Rocks are up.
As for the Heal Bell Togekiss mentioned before, if Chansey gets a T-Wave off, Togekiss has to waste time Heal Belling, during which Chansey can either Toss or Wish... or use that chance to switch to something that can beat Togekiss, such as the aformentioned Zapdos.
Remember, I'm saying you need to look at the bigger picture for Chansey. Don't just look at Chansey and what beats it 1 on 1. Chansey has
FIVE TEAMMATES, all of which can effortlessly plug up the few holes that it can leave open, and Chansey can heal them up later in the game, just in case your Chansey check switches out of them. Zapdos dosen't care about SR so much when switching into Togekiss, when Chansey's wishes are giving him 75%+ HP at the end of the turn.
The teammates support Chansey by beating -Insert Fighting Type Here- and Chansey supports them with obscene Wishes.
... And, to be honest, Zapdos + Chansey walls basically everything but Mismagius, who can be handled by throwing something like Bisharp into the mix. Just to illustrate how easy it is to plug the holes Chansey leaves open.
The only saving grace about Chansey is that it's such a dead stop to Special Sweepers that if you can catch it, and defeat it [Something like Physical Mew might work for the surprise factor, especially if it has Cheer Up to set up and still make Special Offense Possible, or maybe cripple it with Psyshock Alakazam], then there won't be anything left to stop your Special Sweeper, because 99% of the teams only use Chansey to stop special attackers.
Although can you blame them?
EDIT: I forgot Missy often runs sub when I posted this earlier, however, the point still stands upon switching in, and, as Sub Missy is somewhat common, a Missy check can switch in as Missy Subs, so all of which Missy acheives in the end is forceing Chansey out, possibly passing a Wish, while Missy loses 25% of it's HP, more with SR, and may have been statused on the switch.
Although, admittedly, not a lot can stop Shadow Ball + HP Fighting, but Zapdos does a rather good job at that anyway, as it can easily tank Shadow Ball at +0 [With the situation being that Missy subbed on the switch], and if Missy dosen't have a sub, won't like Thunderbolt.
