Hi there,
Frankly most people on this forum are very much 6 series devotees however most are reasonably pragmatic, all our cars can suffer issues....they are after all cars....
In my case, I have a F13 650 Coupe the LCI - 2015. I bought it 2 years ago and touch wood have never had a days trouble with it. It had perfect BMW service history, about 24k klms when I got it and was very well looked after by the previous owner. I've done another 20K klms now and I find the car just as delightful and fun to drive as the day I bought it. I've got rid of the awful runflats and put on Michelins Pilots which transform the ride and comfort.
There is conjecture about the N63 vs N63TU V8 , the N63TU being a much more reliable and sorted unit than the original N63. most cars from 2013 onwards got the TU version. these engines do use some oil but not excessively. I'm not a mechanic just an enthusiastic owner. I don't drive my 650 daily now, I've since bought an immaculate 2010 Supercharged V8 Range Rover again for this purpose. my 650 is now my weekend toy. you can see I'm glutton for punishment…
All brands have their 'horror stories' and we can talk about these forever.... My advice would be buy the best you can afford and yes the later models, 2013 onwards, seem to be fine. Change the oils and service every year and use PREMUIM fuel!. Hopefully the car will have good and clear service history, if it doesn't look for another, while rare they aren't unicorns.
happy hunting !