I can see some of your points. Let me address a couple of them, though.
Firstly, 'displaces real jobs'. That's an argument only if you accept that the jobs were going to be created anyway.
Secondly, the minimum wage is damaging to two groups - the unskilled and small businesses. The higher the minimum wage goes, the more damaging it is to those two groups. So I'm in two minds as to its value.
Thirdly, the wage bill being paid by the taxpayer - if you're in favour of government intervention in any way, there can't be an ideological objection to this. If you think government should be 'stimulating demand in the economy' for example then you really can't object to workfare on these grounds. As it happens I'm not in favour of government intervention so I do agree with this point. However, somebody has to get these people working again since so many of them are unemployable.
Fourthly, employment legislation. I don't think that's a worry because anyone in the workplace is covered by legislation.
Finally, 'international law'. That's a pretty weasel argument, IMO and I'm sure the lawyers would love to fight it at £180 an hour. But nobody's forcing these people to take the taxpayer's money. But that argument of mine comes down to the value of welfare and its failings and is probably for another thread.