The main upside is it gives you the freedom to invest in yourself without worrying about juggling freelance clients.
Want to travel? Long weekends are the norm, and it takes less pto to get extended vacations (particularly if your boss lets you shuffle your schedule between weeks).
Want to create a small business? You now have two days a week to do exactly that. This protects the weekend, and has saved me from the burnout of 'night & weekend' projects I've done in the past. My business is not ready to support me, but it is much farther along than any previous venture I've started.
Want to do random educational activities? I've taken up bread making and taken several woodshop courses in the past year. I also read 50+ books a year. You have time.
How do I negotiate this? I get a salary and health insurance. I do not get equity. I prefer it this way, as I'd rather bet on myself than equity in a venture backed startup. Just my two cents.
Are there downsides? If you consider missed pay a downside, then yes there is. You won't be able to buy a house in the bay area. You won't be able to keep up with the jonses. You won't have a fancy car. If that matters to you, don't take the pay cut. Personally, anything above $70k a year is gravy. I'd like to maximize my free time after hitting that financial milestone to actually live my life.
Let me know if I can help with specific questions.