I've worked at multiple startups sub 50 to >1000 (unicorn status) and have founded and sold my own company. I've also worked at a few of the FAANGs. This is common advice, especially among VCs since it's one of the key benefits used to pitch prospective employees on giving up that fat FAANG salary, but in my personal experience, it's not so cut and dry.
Working in a startup will brand you as a startup person and help you meet VCs which may make it easier to raise. It'll also show you the problems that that particular startup faces and give you n=1 degree of pattern matching. On the flip side, something like YC can give you the same benefit or just shipping a for-profit side project on the side. Additionally, given how poorly run most startups are today, I'd posit that you're actually in more danger of learning the wrong lessons than the right lessons by joining a startup. At the end of the day, the truth is likely to be Peter Thiel's observation that joining the right, highly successful, high growth startup as a very early employee is a great way to learn exactly 1 method of starting a successful company--the way that company did it. But if the startup you join isn't that rocketship, you've only learned how to (or how not to) achieve that particular path of failure or mediocrity.
Not particularly great advice though, because then the challenge becomes identify those startups. And if you could do that reliably, I'd suggest you switch careers into VC instead :)
Working in a startup will brand you as a startup person and help you meet VCs which may make it easier to raise. It'll also show you the problems that that particular startup faces and give you n=1 degree of pattern matching. On the flip side, something like YC can give you the same benefit or just shipping a for-profit side project on the side. Additionally, given how poorly run most startups are today, I'd posit that you're actually in more danger of learning the wrong lessons than the right lessons by joining a startup. At the end of the day, the truth is likely to be Peter Thiel's observation that joining the right, highly successful, high growth startup as a very early employee is a great way to learn exactly 1 method of starting a successful company--the way that company did it. But if the startup you join isn't that rocketship, you've only learned how to (or how not to) achieve that particular path of failure or mediocrity.
Not particularly great advice though, because then the challenge becomes identify those startups. And if you could do that reliably, I'd suggest you switch careers into VC instead :)