The drawbacks:

Different tanning and dyeing methods, on different leathers produces a variation in color from one black harness to the next. It’s not unusual to find harness that runs when it’s wet. In some cases this might happen from time to time when the harness is new, and when the excess dye has leached out, the problem goes away. Unfortunately in some cases, as that color runs, it leaves the harness less than black. I’ve experienced this in cheap, and top of the line harness alike. Regular applications of black shoe polish will slow the process, but eventually you’ll need to reapply some leather dye. With particularly cheap harness the dye may come off on your hands, and your horse even when the harness isn’t particularly wet. This is a real problem, since everything you own will end up with black smudges all over.

Another pitfall of black harnesses relates to leather quality, and it effects the good and the bad. It takes a lot of dye to get a harness black. So much so that the leather often becomes dry, stiff and brittle. Some harness makers overcome this by using a heavier, less refined leather that can stand up to this abuse. However this means that the harness is not as supple and easy to handle. This leads us to the other pitfall where black harness and leather quality relate. It’s easy to hide poor leather quality under gallons of black dye. This is one reason why black harnesses tend to be less expensive than russet. This is not usually the case with a well known harness maker.