Howdy and welcome to the forum
. My family has only ever used a full collar and hames on our working horses, but we had to because used to farm with them. They would frequently be in harness from shortly after dawn until nearly dark and sometimes they spent more time standing than pulling. From what I've seen, so long as the collar fits correctly and is padded properly, then there is no reason for it not to be comfortable for extended periods of time.
The biggest problem I have with the breast collar type of harnesses is that they are harder to fit and keep in the correct position to avoid soring/pinching the horse (granted I am making this judgment based on my experiences with saddle
breast collar styles...but there isn't much difference between a working harness horse pulling a cart/wagon and a working ranch horse pulling a 1000 pound cow on a rope). With the strap being right across the point of both shoulders, it limits their shoulder mobility and they can't pull as much or as far like that. I believe that's one of the reasons why I've only ever seen the breast collar type harnesses on horses that are pleasure driven for relatively short periods of time.
As for needing different collars...you betcha LOL. We have 2 drafts that we drive as a team. When we first started driving them, the collars fit perfectly. Then, as they began to lose fat and trim down, the collars got a little bit loose. Then, as the work continued and they started to build muscle, the collars got too small. So, we've needed 3 different sizes of collars for them.