The CANDU technology is some of the safest technology in the world because it is developed on “defense-in-depth” technology. Yes, the CANDU 6 has a positive void coefficient that can cause a power pulse, but the shutdown system has been designed in such a way that it can handle it. It's designed for it.
They have two independent shutdown systems, which other reactors don't have. The ACR has been designed to have a negative power coefficient or void coefficient. This will also increase safety, but it doesn't make it any safer than a CANDU 6. They are both very, very safe reactors. The design criteria on them is to make sure they can handle the design basis accidents that have been put forward by the regulator, which are very onerous accidents.