True, it does wall all special sweepers, wait no it doesn't, Haunter. Not only that, is there any special walls that can get through Umbreon? Would it make any difference if it was banned? Probably not.
While Chansey walls 'all' special walls it leaves a hole for physical attackers to dominate as its offensive pressure is poor.
Only problem is, a lot of physical attacking threats can get trapped by dug just as easy as Chansey can.
#BanDug