The problems with a cross breed with a fancy name are caused by
ignorant customers who think they can pick and choose from a genetic pool. Are all your siblings and cousins identical with identical temperments? - I think while they may have similarities - there will be a lot of variety and some you wouldn't choose to live with.
Same with cross breed dogs - you can't choose which characteristics you get.
So the cross breeders who say you can are lying. What else are they lying about?
Do you get to meet both puppy parents and find out what their temperments are like and hence the likely temperment of the puppies. Are the puppies even of the cross the breeder says? Could you tell a cavalier KCS x Bichon Frise from a poodle x maltese puppy?
Most "designer dogs" come from puppy mills. Not all but most. How do you tell the difference and what's wrong with a puppy farm? Dogs in puppy farms are kept in very similar ways to battery hens ie small cages with wire floors, no opportunity to live a normal doggy life - resulting in unhealthy puppies with bad social skills ie they often attack other dogs for no reason making them not much fun to take for a walk. And they cost piles of money after you bring them home at the vet for things like bad teeth, bad spines, bad joints etc.
But the cross breeder will say they're breeding healthier dogs. And when you ask them for help with your vet bills resulting from genetic problems - they won't talk to you let alone take the puppy back like a responsible breeder will.
So a bunch of links to help you find a good breeder. Or if you really want cross breed vigour get a dog from a pound or rescue. As part of your research make sure you get some face to face recommendations from the dog breed clubs. Puppy mills and dodgy breeders can have beautiful websites with lots of happy testimonials that they wrote themselves. Ie those are sometimes complete works of fiction. Some of them will even take your money and deliver you nothing not even a sick puppy.
There is heaps of relevant info on the RSPCA website - not just about pound puppies but any puppies.
Choosing a puppy | RSPCA Australia | For all creatures, great & small.
How do I avoid supporting puppy farms? - RSPCA Australia knowledgebase
How do I find a good dog breeder? - RSPCA Australia knowledgebase
How old should a puppy be before they are adopted/purchased? - RSPCA Australia knowledgebase
If a puppy is offered at less than 8 weeks old - run away, this is a sign of a dodgy breeder.
Puppy Farms | RSPCA Australia | For all creatures, great & small.
And a registered breeder does not always mean good breeder. Some advertise they're registered but that means they have a licence from their local council, not that they're with the ANKC and would sell you a puppy eligible for ANKC events (pretty much anything doggy in Australia).
Australian National Kennel Council
Note an ANKC registered breeder should follow the ANKC code of ethics. And you should get a puppy with ANKC papers for no additional cost. Ie all puppies from ANKC registered parents must also be registered. They have two registers - Main - it's ok to breed from that dog, and limited - dog is for a pet only and should be desexed. In the bad old days, these puppies might have been PTS.
What is a responsible companion animal breeder? - RSPCA Australia knowledgebase
Rescue puppies (search for puppies in the size field)
PetRescue - find your new best friend!
How to choose the right shelter puppy
And everything you need to know about choosing and training a puppy.
Digital Dog Training Textbook | Dog Star Daily