I've worked at 2-3 (one I left after a few months, I felt very out of my element). Overall, I don't broadly recommend them, and if you're going to do one and it's not your own, I recommend doing it a little earlier in your career since the public / IPO'd tech money can be double your salary (e.g. it's half your comp). For some anonymity, I'm distinguishing these by pre/post-Series B (seed / A is considered "early stage," no PMF; B-C is "scale up", D+ is... something else, I forget).

• One I joined at post-Series B as one of ~50 engineering staff, 150 people total. We got bought shortly after. Everyone was generally nice. The stack was pretty awesome. When I left, it was maybe ~400 people overall.
• One I joined at pre-Series B Startup as one of ... maybe ~20 engineering staff, one of ~50 people? It had grew to ~35 Eng staff I think (unfortunately, not much on my team) and ~80 people and then started to decrease in size (layoffs). Everyone was nice. I think you need much more than nice when you don't have PMF / do not like your product.

I would agree with @totallynotaspy@lemmy.world. I came into the earlier stage company thinking it was "Big Company Things" vs. "Small Scrappy Startup" things. I had about as much meaningful authority at the chill, scale-up company and with no on-call. But at the earlier stage company, I was not able to propose large work, and I was also on-call when the 5 years of tech debt would show it's age. Improvements were shelved in favor of Sales' vision, because there was no product vision. And even things that were obvious like improving performance on a mission critical offering (e.g., something that would severely impact the customer) were floated around, but were too difficult to budget due to ostensible increases in infra cost and the opportunity cost of not delivering cute features for specific customers. It was a strange kind of stress. Overall:

• You definitely learn how to work independently (though I imagine there are better ways to learn that.)
• Your experience is super colored by the industry, imo. Direct to consumer takes more heat, imo rapidly changing industries with constant trends and pretty Unicorns also have more chaos (AI, blockchain). The two small startup guys I know work in a FinTech and IAAS company respectively, and I was surprised to find out their WLB was so good despite the size.
• Your focus is delivering fast, not doing stuff well. It can be good for learning, but is not good for quality.
• It can be really easy to work your ass off and also lack meaningful resume boosters because the scale of the system or traffic may be relatively low. Make sure you're always working on something that you can spin and talk about in your next interview.
• You have a lot of financial barrier. For example, if you're used to things like Kafka, get used to a cheaper or easier to manage alternative (SQS? Kinesis? one of those)
• Being in a stressful environment for a long time warps your perception, and the idea of a stressful environment differs for everyone. I found it stressful to be so close to a product without the ability to impact it; other people were fine just saying it wasn't their problem. When I left, after a few months, I felt enormously better and different, and my social life had gained new dimensions (not new friends, just-- I could engage with it better).
• Startups are flat and without structure. Your best opportunity at getting promoted is at a place with a promotional structure and with layers of people.

Despite all I just wrote, I am looking to join another startup, in part because I'm not great at interviews, and because I've realized how highly variable the experiences are. I just spoke to one company where the founder was obviously passionate and opinionated-- maybe even high strung-- but had strong vision and seemed to be moving at light speed. Joining would be great, but explicitly, engineers would have little weight on the product roadmap and already some measurable tech debt. Another is mostly engineers and hiring for engineers who are interested in the product space. it seems leisurely but meaningful with a product I liked.