The big advantage of boars hair brushes is that they release dirt into the rinse bucket better than anything else and that is a major factor in avoiding scratching. Microfiber is the opposite. I'm not happy with the mf mitts I'm using and I want to get one of the Carpro merino mitts asap. I also want to get some of the grout sponges that people have got from Lowes - but I need to arrange some way of shipping them overseas that doesn't cost $150.
Someone once mentioned that it was important to use the brush vertically so that the fluffy tips of the hairs are drawn across the paint, not the sides of the hairs.
The softness of split-end boars hair varies. I suppose that the Montana ones that autogeek sells are better than anything else. I tried a roof paint boars hair brush and it seemed ok to me. Certainly, it was easier to use than anything else and there was no doubt about the rinsing. Some time later, I tried to see if there was any fine scratching on my light silver car, with halogen work lights and my reading glasses. I couldn't make up my mind about it, I wasn't sure what I was seeing. What I thought might be microfine scratches...it was only possible to see them by turning my head to different angles. I didn't go as far as using a magnifying glass. That was a smaller brush that I already had (unused). The big one I'd bought for the car washing, I'd covered the metal part with rubber and the adhesive wasn't ready when I wanted it. When I looked at it later, It seemed that the hairs were not as soft was the other one, so I didn't use it.