> IMO the ability for individual employees to negotiate for themselves is a positive?

I keep hearing this, but FAANGs don't allow individual negotiations. You are banded, like you would be at a union.

Also you're assuming that unions would be able to, or want to block the firing of bad performers. Since the bad performers would also hurt the bottom line, and therefore your pay.

Unionisation might hurt the startup as it would stop certain levels of exploitation (ie not being able to ask people to work for free in exchange for shares that will be worth nothing.)