The database will be hacked and all records leaked within the fist 18 months of operation
You know, the same information they need for most people's taxes is already in the IRS database.
I'd also like to say that I've had automatic filing for my Norwegian taxes since moving here to Norway, and always have to file American taxes manually. The emails get sent to a secure email system which you can check. Letters get lost and stolen as well, by the way. The system doesn't have to go down either - I don't even have to interact. I just get the email telling me I can check taxes, but it is not required that I do. (Safer to do so in case of owing). They tell me I'll get my money in a 3-month timeframe. It goes to the same place paychecks go to, though I imagine if anyone gets checks they take longer, as do the people that manually file.
I'm also pretty sure that the reason it would be expensive is that the government isn't exactly known to keep the IRS fairly current and the government isn't really set up to handle the digital age well. But then again, the records are already there. They already plan on getting refunds. And easier filing means more people do tax returns... and it catches more people that owe. Oh, and you won't need as many people working at the call centers because fewer people will need to actually calculate their taxes. I'm gonna guess it would be a net benefit and profit within a very short amount of time.
The main people that do badly with this sort of system are the tax filing companies along with, I'm guessing, some accountants.
Weak security: 
Cost overrun: 
Inability to design a highly available website From :
> The basic architecture of the site, built by federal contractors overseen by the Department of Health and Human Services, was flawed in design, poorly tested and ultimately not functional.
I'll grant that voting works only sometimes. And when it does change things, it's often not everything we want changed, and definitively not fast enough. But it can change things. If it didn't, voter suppression would not exist. (One of the ways of suppressing vote is promoting voter apathy, by the way).
The only sure guarantee is that not voting doesn't work.
Even something as seemingly mundane as voting carries a lot of risk of physical violence in some places. That's how voting was in the US in the 1870s, if you were black. It might sound like a far away past for certain US citizen, but I (an European) own a house older than that.