Not to be a pain, but I have to disagree. I see dozens of dogs a day, the purebreds and designer breeds have more chronic issues - as an example it is 100% known fact that boxers as a breed have a higher chance of dilated cardiomyopathy than random mixes (dobermans and Irish wolfhounds too). A golden-doodle is more likely to have allergies than a jack russle/poodle/beagle/lab mix. All toy breeds have much higher chances of dental issues - because they have been bred to be small - genetically as canines they are trying to fit the same number of teeth into a skull that nature never intended. Therefore the more breeds you throw into a genetic make up the less likely genetic issues are to be expressed - it's called hybrid vigor.
I do agree that mixes can have the issues for their parent breeds - which is why if you have a "white-footed" mutt that develops cancer, you should have it genetic tested for the MDR1 gene to ensure it can tolerate certain chemo-therapies and other drugs. The MDR1 issue stems from collies and other herding breeds. I have met and treated very sick mutts too, it's just not as common for the chronic issues that I see in other breeds.
Again: no disrespect, but I have this talk with clients on an almost weekly basis - hybrid vigor will win out over line-genetics more often than not