Bitcoin has no dispute/chargeback mechanism in case of error or fraud. That inherent unsafety trumps just about all other safety concerns for a practical payment network.
In case of error you talk to the seller, if they are reasonable and care about their reputation, they pay you back (e.g. the Square seller devices have options for this). If they don't, you can try suing them.
There's a wide group of people that are stealing from sellers (especially on Amazon and similar) by using the product and then returning it with made up issues. Bitcoin would help these businesses, since the settlement is final.