Yes, working a timid dog is certainly a huge challenge and is extremely tiring. Such dogs invariably have very low drives and so the handler has to compensate for that in order to bring something out of the dog.
Where most dogs will forgive handler mistakes, the timid dog isn't as forgiving. The wrong word spoken at the wrong moment or in the wrong tone, or the wrong noise made in the surrounding environment, invariably sends the timid dog back to square one, killing off weeks of work and progress with it.
While the usual rules of training still apply to a timid dog, they have to be applied on a MUCH slower basis. If I had to apply a number to it, I'd say it's about a 3 to 1 ratio. What might take a 'normal' dog a month to learn, will take at least three months for the timid dog.
But I will say that working a timid dog is a brilliant foundation for training other dogs. If you can bring the best out of a timid dog, training a 'normal' dog will be an absolute breeze. I home-board other dogs and I love the break of training with them. It's like having ten new gears fitted on my bike! Everything is faster and easier.
Still, we don't get to choose how they turn out and all we can do is keep on going and building the dog's confidence through exposure to as many different situations as possible. That is why I use the fitness-ball to teach her balance and to teach her that I can always be counted on to catch her and keep her safe.