This is a common misconception.

If the value of your invested RSUs go up: fantastic now you make more.

I’d the value of the invested RSUs go down: no problem, you evaluate if you want to wait to see if they go up, or if you want to go into the market and take a new job.

It’s nearly all upside for the employee (in a hot market).

It's not a "misconception." I may not want to go find a new job in 6 months because the stock tanked. There's opportunity cost if nothing else. While that company's stock is tanking, I could instead be at one that's growing at even a modest rate.

While true, frequently cash-rich companies account for this. There are a lot of levers CFOs of profitable companies have to prevent people from leaving when the stock drops: issuing more shares, cash bonuses, etc. Facebook and Apple have more than enough cash to retain their talent in the event their stocks get cheap.

Still, they’re likely to readjust your comp back up to baseline.

You face the opportunity cost of not working for a company whose stock will rise fast, making it worth much more in years 2-3+ than it was initially priced at.

None of us have a crystal ball though, so it’s all speculation.