I joined a series A startup years ago for none of your "valid" reasons and have never regretted it. It was a company that I already knew of and admired their product, and they had enough traction at that point to offer at least several years' worth of runway/stability ie I didn't have to worry about my job disappearing unexpectedly (at least not more so than any other California job). Work-life balance was always totally fine. I learned a ton right from the start, was able to make a big impact, and worked in the smaller "get stuff done" environment that I prefer over bureaucratic organizations.