It is becuase of how tempered glass SP are made for this phone. There is a pretty sizable air gap between the glass and the screen itself. Typical TGSP doesn't have this and have adhesive on the entire back of the TGSP. TGSP that is made for this phone (atleast those that I know of) only have their adhesive on the border of the screen protector. Causing some people to have peeling issues too.

It doesnt effect the phone itself but water might get between the TG and the phone screen hence u might be staring at a puddle of water that u cant easily remove.

Yup

The normal PET films have no such issue, one of the best one on the market is the Spigen curved crystal. Avoid TGSP at all cost especially if you are going to a case.

I went with spigen curved hd SP to protect against minor scratches.

Yes, in comparison, Samsung service centre charges RM89 for a pretty crappy one In addition to that, it come in a pack of two in case of any screw ups.