I've used MTG with quite a bit of success but you have to remember to put it on a couple of times a week-not on the hair, but directly on the tailbone.
In stables where tails are prized, those tails almost never have a real brush or comb taken to them. The hairs are separated out , very carefully and slowly by hand, especially after washing and only when bone dry. Just like in humans, wet hair is weak hair and brushing/combing will break a lot more hairs.
Never had any luck with tail bags, my horses could usually have them off and pounded into the ground within an hour or two!