Do you have a hard time training you dog?

Nonsense.

Depends on the dog and the owner. The owner needs to find the training method for the particular dog. Some are play motivated, some are food motivated, some are touch motivated.

Start simple, keep the sessions short and quick pace. Only reward for any attempt for the behavior. NO PUNISHMENT.
Work on the 7 basic commands, but you need to start with the dog responding to you. This can be accomplished with training the "Look at me" command. Keep your command word simple. In this behavior, I'd use the word "Look".

FWIW, there are numerous mutts that are highly trained. These are not "pure blood" animals, but a mix.

The only way in which "blood" matters is that some breeds are more independent than others, and so mixes of those breeds also will be. So you see a lot more of the breeds that are more devoted to pleasing humans and working with them (like say, Golden Retrievers) in the competitive obedience rings. But that doesn't mean all breeds aren't trainable, just that some are keener than others to do the kinds of things we humans tend to want.