Most of the time, I've never had a real problem with them, in fact, the ones that my local tescos has, are pretty good.
However, my main gripe is with the carrier bags, which are smaller than usual, and due to the way they are packed and hung, are a pain to open, during which time the machine is complaining at you.
I also agree that the scales should zero when you start, allowing for other bags, I use bags for life, and it is stupid that I have to scan everything, pay, then pack into bags, all because, if I put my bag on the machine, it freaks out.
And finally, on the subjects of limiting how many items.. as far as I am concerned, if you have 1 basket, and all your stuff fits in 1 basket, then fine, scan away.