The problem I find with Google Maps traffic is that it tries to be too clever and guess what the traffic might be like when I get to it. My navigation usage is for long journeys so this is particularly unhelpful in the event of an accident that closes a Motorway for a long time - Google happily sends me towards it because 'I'm sure it'll be fine in 2 hours time' even though you know it won't be. Meanwhile, the BMW system just says 'It's shut, go this way instead' which I find more helpful.
Google is, however, much better for shorter trips in city areas. But that isn't really what I use navigation for and therefore I've found the BMW system well worth it for me and good value at about £50 a year. It's also just so well integrated with the rest of the car. I do get Google Maps information on my Head Up Display with iDrive 7 but it's much more basic the the detailed junction layouts you get with the BMW system.
One criticism I have of the BMW system on iDrive 7 versus the older system I had in my 2015 5 Series is that previously I'd get a total delay duration for a journey, at the bottom of the screen, at all times. This was really useful at drawing your attention to an issue - for reasons I can't understand, iDrive 7 won't do this