My goal is not rhetoric or strategic messaging or whatever. I can reliably point out that when systemd was launched, the first blog post describing the project pointed out the lack of copyright assignment as a advantage when comparing itself to upstart and it is clear that developers involved view it as such. No doubt about that and that by itself establishes the negative impact.
Whether patches submitted to a project were rejected on the basis of such a requirement is not a very interesting way to measure the impact as far as I am concerned because anyone who looks at contributing to a project and realizes there is a requirement to assign copyright would just not bother submitting any patches in the first place. If you are interested enough, you can talk to the developers involved and find out more. I have but there is no public reference for you so you will have to do the leg work yourself to find out more.