Happens. I worked at a startup that offered to triple my equity if I stayed a full four years, a special offer to me for seven months of excellent performance. Four months later, growth below projections, they laid me off 19 days before my 1 year cliff. 3/9ths of the engineering team, gone. The lesson I’ve learned is to never, ever, put long hours into a startup you don’t own. Ever.
The thing is even if you 'have' the equity share as an employee, you might never get liquidity. And (super obviously) most startups fail.
So even if you have a decent chunk of 'ownership', you really, _really_ need to believe in it to be working those hours. I'm talking the kind of belief built on your specialist and insider knowledge about why it's very likely to crush everything else. And even then you can get screwed!