A great majority of stories on here are trash without involving harems at all.
That said, it's not the topic, but the writing/story that defines if a story is good or not. I, personally, dislike the way that Webnovel, and other's of a similar ilk, encourage authors to churn out stories as fast as possible. This simply encourages bad writing.
Even if I write a novel normally, it needs work once done. Sometimes I change something in the story which means I should go back and edit something from earlier. WN's format means that editing/proofreading is almost discouraged, lowering the quality of even the few decent reads on here for the most part.
Yes, before you ask, I have a couple of books on here, but one was fully written beforehand, and the other I've been writing as a spoof, although it's turned out quite well, if I say so myself. That being said, the version on here is basically a first draft. Once it's done, I will be going through it, editing, re-writing it to make it properly 'readable'.