The problem with plastics is that they encompass such huge range of materials that you can't really make any reasonable generalizations for them. The most you can say is that they are mostly hydrocarbon based, which is almost completely unhelpful.
Indeed. My 1999 Crumpler is sticky, as are similar things from that era, but the clear windows on my ski jacket’s helmet cracked into pieces. My old 1980’s Walkman case did both on the same case with both materials as you suggested.