When you've finished your story, how do you know if it's any good? Received wisdom suggests joining a writers' group to get impartial feedback from your peers. I've belonged to a couple of online ones and they can be very helpful up to a point.
But the problem is the whole business is so subjective. What I love could be dead boring to someone else, and vice versa. I once tried to review a sci-fi novel and couldn't make head nor tail of it, but then I don't usually read much sci-fi so is the fault mine, or theirs?
And these groups are peopled by a wide array of levels of expertise. From relative beginners to, if you're lucky, industry pros and published authors. Not all the advice on offer is worth the trouble. And who do you believe?
There are definitely some unreliable types to beware of. The embittered wannabe has been writing his whole life and never published a thing. He sees himself as the angry young man, although he's now well into middle age, and the publishing world as a greedy heartless machine intent on destroying his soul so he would rather starve than compromise with these monsters. The pseudo-intellectual who writes "literary fiction" that is so dense and pretentious nobody but him can make any sense of it. The perfectionist who never gets past chapter three before going back to rework the start. Criticism from these types can do more harm than good.
And because of this diversity flame wars can break out on these sites over crits. Some people cannot take criticism and react as if personally attacked. It can get bitter and nasty. Writing is such a personal thing, it's hard to take personality out of the equation altogether. (I'm more a sulker, really. Too much of a wimp to start an argument.)
There is often a trend towards conformity in these groups. Strong characters set the standard and everyone else falls into line. Before you know it your writing is being influenced by this trend and you're losing your individuality.
So, I would say join online peer review groups and take from them what you can, but beware of taking any critiques too seriously. Ultimately writing is a solitary activity.