I am not familiar with the Koester issue, having never owned an example, but I had great success soaking a group of E121's, mostly 80 series, a few years back. I also have had E120's that were a bear and caused back damage havoc when I soaked them...I think it was the glue used that caused the difference in how they soaked. Like Rhett has mentioned that ink writing might transfer onto the cards. The E121's I soaked were pasted onto red-lined ledger paper, and well, the cards now have red lined backs. Fortunately they were all trimmed, so those red lines almost add to their overall appeal.
Brian