A flat tax on consumption is fine as long as you do not dig too deeply in it.
The main purpose of taxation is to provide a global pool of money to accomplish tax that can not be efficiently performed by each taxed entity.
You do not expect each citizen to build the 10 yard of road in front of his/her house. You pool money in, and the the global entity (government) will pave the road. Flat taxes are a reasonable way to gather fund for activities that touch all part of your country, like education, unemployment funds, defense spending.
The problem with a flat consumption tax is that it does not allow you to tax more activities that causes a particular stress on particular aspect of your society.
For instance, if two modes of transportation have widely different impact on the cost of maintenance of the globally owned infrastructure, you want the cost of that maintenance to be attached to the entity, goods, that are using them. If flying cost to the company $1 per product, and driving cost $2 per product, then the companies will fly. However, if the maintenance cost of flying by the government is $10 per product, while driving would only cost $2 per product, this is a catastrophy because the government bleeds money for each product flown. Because the real cost of flying does not appear in the company balance sheet, the company will not care. So you need the real cost, or at least something that looks like the real cost to appear in the companies balance sheet.
That is why we see things like taxation on gas (to recoup some of the road cost), or taxation on electronics (to recoup the cost of recycling them when they get to the garbage), or airport tax (to recoup the cost monitoring flights, the runway, security, ...).