I think it’s important to bring up that at times when people give their opinions on being rude vs making constructive criticism outside of this, it’s often really about what people think should be done when it comes to addressing problematic stories and themes. But to be honest, even that misses the point.
If I’m publicly criticizing a story for the mishandling of serious subjects or something similar, my focus really isn’t on giving feedback to said author - at all. Mainly because (a) I’m more interested in talking to the “third party” so to speak and (b) almost every time, if not every time, it’s obvious that the author is not open to fixing any of the issues.
In that situation, I’m not giving constructive criticism persay nor am I trying to. It’s simply criticism. My focus is on communicating to the third party that something is wrong and we shouldn’t be doing it ourselves or letting it sail by without addressing it.
That being said, I think the most important thing about giving feedback in a review is: whatever you do, make it your goal to help the author get as close to their vision for the story as you can.
Part of that is making it a point to give the person requesting feedback as much as they asked for - no more, no less. So if they ask just for critique about their pacing, you only look at pacing. They ask for feedback on characters, you only look at characters. If they want everything, feel free to bring up anything that stands out to you. That isn’t a hard and fast rule, but the idea is to not have it in your mind that you need to fix everything about a story that doesn’t belong to you.
General life advice: be wary of anyone who prides themselves on being “brutally honest” in all situations and be wary of letting yourself fall into the trap of it too. Honesty doesn’t need to be brutal to be fully truthful. Even if a story couldn’t possibly be more flawed in all its technical areas, it’s important to value compassionate honesty and word your feedback with their best interests in mind. If you can do that, you’re well on your way to being the most intelligent and thoughtful reviewer and/or beta reader you can be.
Likewise if you’re a writer, you need to learn to separate your sense of self-worth from the stories you write. Because if you don’t, even an innocuous comment about poor grammar can cut deep. Writing a bad scene or having a bad idea doesn’t mean you’re a bad writer. And even if you are a bad writer, you can get better. If someone is genuinely giving you good feedback and seems to be looking out for you, lashing out is a mistake and it’s wasteful of all the effort they put into helping you.