It would appear that the sealed bearings were not as well sealed as one would like.

The stock bushings in most derailleurs handle extreme weather just fine.. the sealed bearing models and after market jockey wheels do make things pretty smooth but so does keeping the stock pulleys clean and lubed.

Suntour jockey wheels were very well made and can be used to replace some other jockey wheels... they do not wear out as quickly as Shimano jockey wheels either.