Assuming you can sell the stock immediately after it vests (usually every 6 months), there is not a downside. Basically you're putting money in an account to buy company stock at a 15% discount at the end of the 6 month period. So when you sell it, you should see an immediate 15% profit on that by selling it immediately. The exact amount may be slightly higher or lower depending on the specific stock price it was bought at vs the price when you sell, but if you sell the stock the day you get it at market open then that difference will usually be very negligible.

Yes you have to pay some taxes on that 15% profit, but that just makes it a 12% net profit. If you can deal with the cash flow reduction for the first 6 months then it's absolutely worth it because it's free 1.5% bonus (assuming 10% of salary with a 15% stock price discount). After the first 6 months then you can just use the previous money to "pay yourself" to make up the gap, and then there's not even a reduction in your income. It's also a nice enforced savings mechanism.

Most of the negatives are only pertinent if you're required to hold the stock for some period of time before you can sell it. Or if you just decide to hold the stock for some reason and don't sell immediately.