I got my first negative reader feedback on Sand and Blood. But, it was probably some of the best opinions I got so far. The reader in question stopped reading at a certain point and then went on to tell me why they gave up. It included the themes that were triggers for them and the aspects of the culture that bothered them the most. It was, in other words, exactly what I was looking for.
I already know SAB isn't going to be for everyone. I don't always write fluffy stuff and I went with a survival story closer to Lord of the Flies than Harry Potter. This was also focusing on a culture that is relatively intolerant of weakness (inspired by 300) and willing to risk death for even teenagers to determine who they really are. Yeah, they get magical powers out of it, but it isn't... fluffy.
Later, I found from their spouse and from them, that they were afraid I would be upset with the response. They didn't know how much I struggle for honesty, but even I wasn't expecting to have such a thoughtful reasoning in the apologetic email. I usually don't get any response when people give up. It just goes off into limbo and leaving me in doubt.
While I'm emotionally invested into SAB, I'm capable of separating myself from my work. I will never be upset about well-reasoned, honest feedback. I can't be a writer for everyone, but knowing why people set it aside is utterly fantastic.
One person doesn't like it? Might be an opinion. Two people don't like it? Might want to look into it. Twelve say it's wrong? Just fix it.
A good motto for writing, along with:
Just keep swimming.
Right now, SAB is with two more beta readers (one I honestly do expect to just stop reading) and I'm still working at fixing some bit-rot to get the print copies to two more (both friends or family).