I have never taken my dogs to a formal trainer. Luckily I have never needed any, having my boys since they were puppies. I trained them myself using treats and not using them. House training never got a reward of treats, just a good boy and a lot of petting. Other things like sit, lay or speak for example always got a treat.
I do thinkmyou can overload on treats. Not just the over indulging of them, but maybe the dog not behaving or doing what is being aked of them without getting a treat could be a problem.
I try and give my dogs treats a couple of times a day, for any reason not just because they do what I ask or behave. I don't want them to rely on treats to do what's expected of them.