There was a warning given in my last box of Tetenal E6 3 bath that some plastics absorb chemicals and will affect the process. I have always used plastic tanks and measures made by JOBO and have never had a problem. I think it is the Company covering themselves in case it actually happens or is blamed for something going wrong and they are sued. I wouldn't worry too much.
There are many types of SS some are more resistant to corrosion than others. Sounds like PF is trying to cover their ass with this notice. If the tank and reels are thoroughly washed after use I see no real problem.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.
Most stainless steel tanks and reels are made of 316 stainless steel, which is resistant to most chemicals. Black and White reversal processes generally use sulphuric acid. In the MSDS for sulphuric acid, it says that it should be kept away from 316 and 304 stainless steel, the acid will eat the steel if the concentration is high enough.
As long as you are pouring solutions in and out you are okay. The old dip and dunk requires more care, as chemical reactions are time and temperature based and you are letting the chemistry sit in contact with the vessel.