Here is a variety of harness that I have personally made. I will start with some information about webbing - nylon and polypropylene are not the same thing. Nylon is treated differently in the manufacturing process, having a chemical added that makes it soft and "slippery" - polypropylene does not. Some pretty harsh, and extremely incorrect data being said about webbing harness - that it "soaks up pee, and poop, sweat, and dirt - get's stiff - rubs and hurts horses". I guess if somebody threw their webbing harness in the horse pen, and let the horses walk all over it, indeed pee and poop on it - then they picked it up, hung it on the fence, let it dry filthy, and stuck it on their horse...perhaps in that situation, it would rub or hurt a horse - but thankfully, nobody does that. Webbing harness such as mine, that is padded with marine grade vinyl will not hurt a horse, and indeed, nobody has ever actually shown a photograph of an injury caused directly, and only, from use of the webbing harness. Poorly fitted, or poorly designed harness - made of any material, now that, hurts horses.
I use neck collar harness 99% of the time - and have for years, promoted the Coblentz collars. They have been properly downsized for the minis for years, are made with high quality leather, are correctly stuffed, and shaped, made as a full neck collar should be - no funky stuff done to them - they have always been the best on the market for minis.
When I do make breast plate harness - I make the breast plate very specifically. Only a slight curve in a breast plate is "required' - and I have seen over and over again, breast plates of several different styles and designs, that do not either fit the horse correctly, or are indeed shaped in a manor that does not allow for proper pulling efficiency for the horse. I have buckle-in traces, but I never, ever, put a ring on them, as that construction drives the neck strap up-tugs forward to the horse's chest. If one then tries to get a bigger one for the horse, the wide end of the breast plate ends up too long, and then though the neck strap sits where it needs to sit, the breast plate end then jams into the girth line of the harness saddle girth, and over-girth. There is no need for a ring on any breast plate - as an "adjustable line of draft" is a myth, only perpetuated by some in the industry.
Last, but not least, we drive with our harness everywhere - road driving, on my lawn doing cones, and indeed, on trails - any harness can be used to drive on trails - what is important regarding trail driving, has to do with the cart or carriage - and harness only comes into play there in terms of fit, function, and optimizing the horse's ability to pull. The images are my red and white, collar and hame carriage harness. The lime green one is a log skidding harness - the next three images are assorted single or pairs breast plate harnesses, and the last photo is my Tonka's draft harness - for skidding logs too, but I can also use it on a cart.