They are front loaded. Your employer pays them to manage your 401k. They say there are no fees, but they do have "Managment Expenses" which are the same thing. Those are on top of what your employer pays them. And they do hold money and not let you transfer it. At work a friend had a lot of money in their Real Estate fund. It was going down. He wanted to transfer it to Money Market. They put him in a queue to wait to be able to transfer. They said it could take up to 3 months.
There are a lot better. Avoid Principal Financial Group if you can.