I think that something can be considered to be broken both if it is completely unstoppable or if it shifts the metagame considerably simply based on its presence. The stallish metagame we are in exists in no small part due to the destructiveness of rain teams against standard offense. It is nearly necessary to include defensive mons in a team both because of the power of some of the walls in the tier, but also because of the power of the offense. Milotic is a good example of this. near-broken generic walls are the only things powerful enough to beat some of the better offensive mons.
Also, I have been thinking about a rain team that abuses dugtrio's ability to wear down Registeel, Blissey, Raikou, Scarftom, and other would-be rain counters to allow for some of the more powerful special rain sweepers to go to town. The initial power of the special sweepers is substantially more than that of the physical ones, however they are also slower, and the premier special walls aren't weak to water like the premier physical ones. There is also the no stat boosting moves problem. It could work though.