You don't know how much tax you will pay before you lodge
You always have a tax surprise at the end of the year. You know it must be because of your ESS or ESPP but that doesn't stop the frustration you feel.
You are not putting aside money in advance for the extra tax bill
Because you don't plan your tax each year, you don't put aside or provision money for your future tax. Therefore, you sell shares, which creates more tax for the next year and you are now on a treadmill of lumpy tax and cash flow payments
You don't know which parcel of shares to sell to give you the best tax outcome
If you do need or want to sell your shares, you don't know which parcel of shares to sell for the best tax result. Selling the wrong parcel could make your tax position worse