The original idea of black box insurance was a good one, prove you drive safely and get cheaper insurance however insurers now see it as a great way of making extra cash from motorists who suffer from expensive premiums, mainly younger drivers
Underestimated your mileage - enjoy paying extortionate rates for extra miles
Have to brake suddenly - ruined smoothness score
Not to mention things like curfews and penalties for driving long distances
My advice is normally if you can afford it, go without but if you really need to get a box. have it for a year and get rid ASAP