You pay fees for every single transaction: chain storage, gas, if you want to deposit or withdraw or exchange to another currency, etc; it's slow to go through and at peak times you get gassed out and it doesn't go through at all.

The value fluctuates wildly at all times of day, so your transaction might lose a significant percent of value before you've had a chance to even access it let alone trade it for a more stable currency or asset. Oh, and you have zero safety net for if something is entered wrong, a key is lost, if you end up not receiving the item you purchased, you're outta luck and there's no bank or credit card agency you can petition for a chargeback. Plus you lose ALL your money with no recourse if you can't keep track of a non human memorizable secret key, or if it gets in any way intercepted or stolen from your computer, or if you accept the wrong transaction...