Especially when you are replacing already cheap "unskilled" or manual labor with even cheaper labor.
In the long run markets will adjust but the long run in this case can be longer than the life of a single worker.
Depending on how much labor you introduce you get get into a pretty never ending cycle of rotating your employment force.
And one can make an argument that displacing day labor with homeless people can have a bigger negative affect since employed day labor is more likely to have financial obligations and dependants than a homeless person.
This is why it's very important to carefully manage these programs and ensure they are at best self sufficient with some public funding but never profitable.
You want them to maw the lawns where lawns would not have been mawend before not to take over existing works.