VirginMedia UK nameservers are notoriously poor, they just stop responding or disappear for hours. At least with Google I’m more or less guaranteed a response, as slow as it might be. I’m in an area where VM is the only cable operator, so I couldn’t switch without either losing on speed or spending a ton of money for someone else to cable me up.

I do try to use VM dns, but I periodically have to reverse to Google and often forget to switch back post-outage.