Many of my plants are planted in the Scott's brand peat and they seem pretty happy now...
The lesson I learned was that I always wash what ever media I am using before potting. I think that in addition an overnight soak (at least) lets the harder to disolve stuff get out of the mix, so to speak.
I imagine that if plants are in a drained container, after a few waterings most everything is washed out. That is unless you have some kind of 'contamination' in the mix (fertilizer pellets, whatever) - but that's another story.