Some tough questions
an example could be the Eurasier which once was a "designer dog", of chow chow keeshond and samoyed, though the term designer dog is only a recent inclusion into language.
what specific breed characteristics can one reliably breed for in designer cross breeding that could not more reliably be obtained through pure breeding.
First it was called the Wolf Chow(designer name) then later in the 70's the Wolf Chow (which was the chow chow/keeshond cross) was crossed with the samoyed and named Eurasier( designer name chosen to reflect the dogs European and Asian backgrounds in breed make up) and FCI recognised within just a couple years afterwards, the breed was formed for no reason other than as a companian dog with the good characteristics of the chow chow, keeshond and samoyed etc.
Some work very hard with a goal in mind to create breeds which cement traits and characteristics etc.
If you find yourself going through hell; Don't stay. Just keep on going.