Keep one thing in mind when you write a review and you'll be fine - how would you feel if it was about your book? If the wording would be hurtful, change the wording.
But that doesn't mean don't criticize. Critique a work looking at ways it can be done better, so the author has a path to improvement - being fluffy when something is bad doesn't help people learn and it doesn't help readers.
There are some reviewers who have seen me as holding a high standard for stories, and my response is that a high bar helps foster excellence - and excellent work is the only thing that will help web fiction to be taken seriously. But no one reviewer has the power to decide what that looks like alone - if you're wrong about a review a fan of the story or another objective reviewer will balance out the viewpoint.
It has happened where a really negative review from a reliable reviewer will cause traffic to a site to increase because people want to see why the review was negative.
I've also had it happen that good writers will request that I review their work after a series of my reviews go up - and invariably their work is solid. When a reviewer's style is consistent, reliable and honest it attracts writers who take their work seriously and appreciate a solid critique.
Hand-wringing about whether you were mean in a review is the same as worrying if your fiction was good enough. The solution is the same - practice, write more, find your voice. People will listen when it is well-developed, but they can't correct it for you.
Practice and improve. If you think that you've been overly negative, write a few reviews for stories you actually like before doing another tough one. And with both, demonstrate your rationale. If you can give good reasons, then over time people will know where you stand.
For example, I'm up front about my biases. I will deduct a star for vampires, wizards, zombies, superheroes, or anything else that comes across as blatantly derivative or trendy UNLESS the writing kicks ass. Anyone seeing me give three stars to a vampire story then automatically knows that if they do like vampires, it is a four star story FOR THEM. If they hate vampires, they know to steer clear too. Consistency matters.