Ask HN: Stock repurchase agreement vs. stock options

Is there a downside of having a stock repurchase agreement over stock options? What is a stock repurchase agreement? An employee gets all her stock grant when she starts and is put on a standard vesting schedule. When the employee decides to leave before the vesting schedule ends, the company repurchases the unvested stocks for a fixed (typically nominal) price (which is agreed upon in the stock repurchase agreement). This seems more beneficial than ISOs, where the employee often needs to pre-purchase the stocks (exercise the options) and if/when she leaves the money spent on exercising the entire grant goes to waste. Definitely not an expert in business accounting and taxation, so there may be huge details I'm missing here. Also, keeping track of these (vs ISOs) could be a bigger burden.