Plastic is, well, plastic. If you deform it, which is very easy, it will stay that way forever. You can't straighten it up. Notes don't stack together nicely anymore, because each note shows a different pattern of wear and tear.
Notes are supposed to be durable, but are extremely easy to chip on a side, and when that happens, they simply split in two. This usually happens with larger notes. When I get money from the ATM, 10% of the notes are chipped on a side and I have to use scotch tape to fix them.
Best note material is what dollars use.
Plastic notes stacks are also much harder to count fast, they don't stick to your fingers, instead they stick together. If they are even slightly wet the problem is exacerbated.
In Thailand, I keep my money roll in the pocket of my board shorts and don't bother trying to stash it anywhere when swimming lest the pineapple lady swipe it from under my flip flops. Paying for lunch later on consists of either trying to dry a bunch of sodden bills in the sun or praising my luck for having a couple (plastic) 50 baht notes that I can give a quick wipe and hand across.