Hi Guys. Peace to you all.

When I try to build an OTF fonts with extended range beyond standard codepage, some characters have absolutely no place in Unicode. When puts them in the Private Use Area (PUA), the font is proven to work well on the two platforms, Windows and MAC. But I've read a suggestion that PUA is not recommended. Even specifically Microsoft Font Validator gives warning the font has mapping PUA characters. It seems indicate that, using the PUA could bring problems. This fact is a bit confusing. The questions are:

1. What kind the problems by using PUA and solutions?
2. If not a PUA, then how to encoding the characters which haven't mapped in a codepage? The sample characters which I let do not have Unicode is "germandbls.cap" (the case-sensitive feature) will being "SS" (when typed in Indesign).
3. Still related to (2). I've read other threads about the PUA on this site, which says that Adobe itself has to accommodate the PUA on certain characters. But of course be understood that the glyph design, will continue to grow, such as alternative forms of the characters in the stylistic alternate. Adobe OTF feature even register until 20 stylistic (ss01 - SS20). What codepage properly for these Glyphs? Here I also encourage the issuance of a design standard guidelines for this issue, from the competent institution off course.

Thank you in advance for your enlightenment.
Best Regards
awm