The biggest downside is the restrictions. If you have to hold it for a certain time it's a larger risk then if you can sell it immediately.

Also if there's any costs. Pretty much every brokerage has 0 fees for buying stock, but my ESPP has a selling fee even though it's with a firm that otherwise is 0 fees. So it cuts a bit into the upside and only make sense if I'm getting enough to offset the fee.