On one hand, it is absolutely stupid to write what you know, especially for fantasy writers. I mean, none of us know what's it like to live in a magic land or to live in sixth century BC Greece. It's all about your imagination.
At the same time, it's hard to write what you don't know. Like...I think I have a mother fetish. What do I mean by that? The MC of my novel is a mother. And I'm trying really hard to make her feelings and thoughts authentic, but what the hey. I don't know what's it like to lose a child. Heck, I have no idea what it's like to go through labor and watch a BABY pop out of your loins. It's pretty hard to simulate something like that. And plus, the fanfiction story I'm supposedly working on at the moment (supposedly, because it's been on hiatus for the past week and a half) is about a mother dealing with her son being a "problem child." He has ADHD and dyslexia, and he attracts all kinds of monsters. I don't even know anybody personally who has dyslexia. I've met maybe one person who had diagnosed ADHD. And I've definitely not met any children of Poseidon.
I'm still pretty young, so there are plenty of experiences in life I haven't had. It makes it all the more difficult to write certain scenarios in an emotionally accurate way.