True story: I worked for a startup that was acquired by a large company, which then went public, and a bit later laid me off (and everyone else in my division apart from a few poor chumps kept around for maintenance). I got more in severance than I got from my stock options.

When you also consider that the same large company took one look at my skillset and responsibilities and promptly doubled my salary post acquisition, it's not nearly as clear what the downside is.