while it is true that Acetate can break down, particularly if stored in areas of high temperature and Humidity, accompanied by rusty metal, it is relatively stable. High humidity can also allow the groth of fungus on any type of film, so keeping the humidity down is always to the good.
Poly film is used for industrial photography, (such as shooting from aircraft, etc) as it is very dimensionally stable and very tough. This is the same reason is is used for Motion Picture PRINT film. On the minus side it is curly, and attracts dust.
I shot an old roll of HP5 Motor drive film from my freezer this summer, it was of course rather fogged but still worked. I used a canon 300V as most of my canons stop after 36 shots no matter how much film they have. I still have the special Stainless Tank and reel, but the filling cap has been misplaced. I managed with some gaffer tape.
Most movie negative is still Acetate.