Pros
Companies are reflections of their founders. Vercel has a visionary CEO who combines deep technical expertise with fantastic interpersonal relationship building. This is reflected in the culture of Vercel, which is very thoughtful and technical whilst also being warm and caring. I'm not here to shill Vercel or say it's perfect. I can only speak from my own experience and I don't doubt it's screwed up sometimes. But it get the big things right. The senior leadership are superbly talented (although it's correct they're not public company leaders with decades of experience – this is a young company). The company as a whole is highly meritocratic and entrepreneurial. If you are looking for somewhere where hard work, intelligence, and creativity will be rewarded, this is it. You won't have your hand held – you need to be able to take ownership and perform. But that hard work will constantly be met with encouragement and appreciation, in my experience. Some of the reviews below criticise Vercel because its values are informal. I personally love this. I can't stand corporate values statements that don't mean anything. Vercel's values are non-cringe and meaningful and the result of that is that people use them in emoji form every day.
Cons
The company is now big enough that your experience will depend to an extent on which team you're on. Make sure that you get on a team with an excellent line manager. This is a hypergrowth company and there are absolutely growing pains. Be prepared to need to take ownership and fix and improve things. Culture can be demanding and perfectionist. This leads to excellent work but expectations are high.