Some rescue dogs can be more difficult, especially if they come from a place that treated them badly and abused them.
Some rescue dogs are the sweetest creatures you could ever hope to meet. Some are not. I suppose you can say that about breeder's dogs too, but the ratio for difficult dogs is higher in a rescue.
If I get a dog from a great home that socialized them and bred 2 good dogs, they will probably be less of a risk for a dog that needs more knowledge from his/her new owner.
With someone new to dogs, a dog that needs more attention and patience is maybe not the way to go. I don't know how to deal with a dog that has more anxiety, or a health issue. But I will learn a lot from having this dog and my next dog will probably be from a rescue shelter as I will be more able to handle its unique needs.