Developers at unicorns: even in the best of times, do you feel more expendable than you've felt at other jobs? We always are amazed at the number of developers at companies like this. (the numbers I've seen are old, but I guess out of 22,000 employees, it was something like 5000 engineers?) While that allows you to build in a more robust way than a smaller company, it seems like there's no shortage of developers work…
Worth pointing out that Uber is no longer a unicorn. It is a large publicly traded corporation with a market cap of $58B. I suppose you're asking your question about large tech companies generally, not so much unicorns? Most unicorns are still quite small (revenue and employee count-wise) relative to large publicly traded corporations like Uber and the FANGs.
That said - to the question about unicorns: You're always expendable. Same as any job - same politics - same nonsense. No matter how important they make you think you are to a products success - they'll fire you regardless. It's amazing how fake management will be about urgency. "This is the most critical business function! We must have people working on it!11!!1!1!" Fires key employee because they didn't like them - "critical business function" gets shoved into the backlog to never be seen again. People are fake and think they need to show urgency to get the most value out of their employees.