The 8 series is a huge car without all the huge car conveniences. For example, if you get the Gran Coupe, there’s only really two rear seats, they’re very comfortable but in a 7 series you can actually seat 3 people properly. The other thing is the boot, again it’s a big boot but the opening isn’t very big.
It’s also a low car, so things like children’s car seats are slightly less easy to deal with than in other more practical vehicles.
I don’t think the maintenance is any worse than anything else in the same range, if you have a b58 engine then you’re in a good place anyway.
If you’re buying used then they are not as expensive as you might expect. If you’re looking for new then I’d lease rather than take ownership.