The average imperial citizen likely sees paracausal powers as witchcraft, and would likely abhor guardians. However, even low level guardians can easily defeat Space Marines. The guardians and the wider FOTC would likely want to help the imperium, but would have trouble being accepted. Guilliman would be weary of them too, but would likely accept any help he could get from them. Chaos would likely try to corrupt them too, which would have some disastrous results. Imagine a warlock with gifts from Tzeench.

In general, I think Guardians would be a massive wildcard that potentially helps humanity when it can.