I'll give my input on this since I am an avid DW OU player.
First off, everyone who plays DW knows, if you don't have something to check Keldeo your chances of winning are severely hampered. Thats how powerful this thing is. It actually steals games. I was playing today, down 4-1 to the obvious hax that took down the majority of my team. Keldeo came in on a -2 Chandy, set up a +1 with CM, proceeded to spam Hydro Pump and KO the rest of his team, with the best thing he could do is Scarf Rotom T Bolt which didn't KO. Hydro Pump coming off of it is obscenely powerful and Pokemon that resist are at risk of being 2HKO'd and non resists basically fall at a OHKO. It will make a large impact on the metagame and become rain's number 1 abuser.
About the topic of Terrakion+Keldeo wallbreaking combo... IT WORKS. Everything is scared of Terrakions CC, so they switch in a check only to be demolished by Keldeo, and vice versa. About Garchomp, the sole reason for its ban was because of Sand Veil. Outside of that it was very manageable. I could bring up the topic of a ban on all Evasion based abilities seeing as not only did Garchomp abuse it in OU but if anyone remembers what Snow Cloak and Hail did in UU...
Garchomp unbanned while every evasion ability goes under the evasion clause. IN DW, Garchomp is only popular because Tyranitar is such a solid check to Chandy, and it is one of the best Sand Sweepers. Prevalence of a check to Chandy caused it.