1. It fails-unsafe. If your external IP changes and not immediately updated in OpenDNS then all previously blocked sites become available
2. It logs all your DNS requests by default.
3. It can be easily circumvented, by simply pointing at a different DNS server (8.8.8.8 anyone?) instead of using the DNS provided by DHCP
4. It’s owned by Cisco. Cisco is known to have had many issues with securing their stuff

A better alternative is to have an internal DNS blackhole, like pi-hole