Aye, I realise that cairncamper
. You are correct, it is coated on the inside with a very un-breathable almost rubbery substance. It works. I've had it a couple of years and it's not let one drop of rain in (and survived a mock ice bucket challenge, lol) but basically, it's like wearing a plastic bag (hence the layering system). As yet, none of that rubbery coating or the seams have perished or delaminated. To be honest, at £7.99 I don't really care if it falls to bits.
If I was heading up in the mountains and/or in winter then I'd go for something a little more expensive and technical. A fashion statement it may not be but it does what it says on the tin and I find it absolutely more than adequate for general walking.
All in all I would say mine is excellently made and it has kept me dry in torrential and prolonged rain (but, like I said, sweat is a different matter and needs managing via a wicking layering system). Perhaps I've just been lucky