Below_the_current I also afraid of giving a review when I found a novel I don't like. But, as an author in this community myself, I tried to think from a different perspective when I need to review someone's novel.
—I have to understand what I don't like about that novel. I need to know, is it my problem or the writer's problem that I can't get into this novel? This thought is pretty subjective, so we need to be careful when reviewing.
—When you don't like the novel, you can focus on the point that you don't like inside the novel. Tell the writer why you don't like it. If the fault falls on you; eg... The novel is not in your taste. Just say it. But, don't say it's bad because it's still the hard work of that author.
If the fault falls on the author, eg. Bad grammar, no paragraph break, messy writing... Tell her that it needs to be changed. Send her tips or something, in other words... Constructive criticism and not a complaint.