I don't know how RSUs count against the 500 shareholder rule -- but for tax purposes, they count as an UNrestricted grant. That is, if you get $1M equivalent shares in RSUs, you get taxed immediately on that value, regardless of the restrictions. So you have to pay $350K (and that's just federal!), which you have no way to recover until the restriction is lifted. Worse, if you get fired the next day, you're out $350K with no way to reclaim them.

Pay attention to tax laws. If you are well off, that is your single largest expense.