I completely agree with Ruth there is a mid-point and end of the spectrum trainers either way are not one I use. I like trainers who aren't afraid to learn something with each new dog in their class and possibly apply it if they should have another one like it again. I also agree that completely positive doesn't work with our dogs either. We use only positive methods to teach a puppy (or adult dog new to the situation) the desired behaviour until we are sure that they know what they are supposed to be doing when they are asked to do it. Once we are sure if they are asked to do something and it isn't done then there is a correction for it. It can be something as simple as withhold a good boy for a lot of these dogs that are really handler sensitive to a pop on the collar depending on the dog.
Here is the way I see it, they are all training tools and what works on one might not work on others. We have dogs you can use a flat on and get the desired behaviour, some you can use a fur saver, and some you can use a prong on. Not all require a prong and not all will give the desired results on a flat. That being said we use the "training" collars strictly when training (whether we are walking in a new environment and need to ensure the reaction happens exactly when I ask it or if we are heading to dog class), while at home they are switched back to flat collars. No matter what type of mastiff you have they can get collar smart to any type of collar so it is important to understand that although these will help you get the control you need immediately, really working on the training to ensure that you have the control with or without a collar. Never let someone make you feel bad for using what works and if you find the trainers aren't the type you can work with then move on and find one that suits what you are looking for. You are the one around your dog daily, you can see which methods the dog responds to and use what works, trainers aren't infallible but some aren't willing to learn once they think they have found the best way.
We are in the same boat (yes I own dogs that aren't perfectly trained
and the love to show it when we are in public at times), we have some smart ass dogs that I still have to be on top of (***cough...Hemi...*** cough... Sequoia...***) and some that would do anything just to get a good dog and all varying in ages so know that whether they are 5 mths or 5 years training is a continuing thing that you will have to stay on top of. If you have an easy keeper maybe not as much as a smart ass but only you will be able to determine that after some time.
Good luck and keep us posted!