Difference between Chansey and Wobb is that you can support your team to take out Chansey for your special sweepers, either with hazards, hyper offensive physical sweepers, or hey even your own WobbTrio! However, with Wobb, each pokemon needs their own individual options to be able to avoid being taken out.
Also, regarding Umbreon, Payback gives you a better chance of taking out Raikou if say you had a few spikes support before he managed to come in, and also prevents him from keeping a Substitue up, allowing you to be able to revenge it. Dark Pulse also doesn't OHKO Mismagius.