Are there any risks from a synthetic bone graft after tooth extraction?

Not much: Mostly just chance of rejection of the graft but even that is not much. Talk to your provider. It is done to maintain bone structure and can be very helpful if an implant is planned.

Minor risks: This is a fairly successful procedure. The risks are infection, loss of graft material or failure to maintain ridge width for an implant. While controversial, my experience is that cadaveric bone works much better than synthetic bone grafts if planning on implants.

Minimal: Synthetic bone grafts are safe. There is a risk of an infection but the risk is also present with any bone grafting procedure.