Often, when people criticize the NHS they have an ulterior motive, like privatisation. Consider all the political difficulties the NHS has had in the past few years. As such, negative remarks can be read or misread as dogwhistles for other views, so they're something that have to be phrased carefully and within context.
I was unclear: did you publish a book, or did your sister?
In general, for something both as key and as endangered as the NHS is, criticism isn't always useful -- support is. Problems can be recognised and addressed through support.