I'm not really sure how options should be valued, and I haven't negotiated from a position of have a ton of unvested equity. But if an engineer got extremely lucky by joining the right startup, say Coinbase, leaving early might mean giving up a few million in unvested equity.
That's a lot of equity if the startup treats that lost income as if the engineer had invested that amount in the last fundraising round. Options instead of stock, common instead of preferred shares, and risk adjustment should all probably increase the equity amount. I'm not sure that most startups would give mid-level engineers millions in equity.