I've had a couple of mega lightweight ones, A posh brand one and an Aldi one. not sure they do much to keep you dry, but do turn a bit f wind and keep you warm.
I got a dhb Flashlight Waterproof Jacket recently, slightly bulkier, but still jackets pocket sized and that is definitely much better at being waterproof and reasonably breathable. tested in some heavy-ish rain and did its job. only downside is it doesn't come with a compression bag or suitable pocket to roll back into, but I had a mesh bag from elsewhere that does the trick.