Reliability-wise, you shouldn't be in for a shock, like someone who'd only ever owned newer, Big 4 bikes. BMW is comparable to "new" H-D, and more reliable than old.
As stated, the biggest issue is also the greatest strength of these bikes; my Daddy would say, "Oh, it's got all that great stuff on it? Just more to break!"
Obviously, most of us think the pluses outweigh the minuses, but you won't find many mindless Koolaid drinkers here who claim these bikes never need repair. The more complexity and sophistication you add to anything mechanical, and the more issues have potential to crop up. Keep up with the service, as also advised above, and your reliability chances go up markedly.
If, like many of us did, you fell in love with the engine, and the incredibly nimble handling for a bike so large, you'll most likely be happy as a clam.
Even if I didn't address it, ALL the advice you got in this thread is excellent. Welcome to the forum, and post often!
2019 K1600GT Option 719 Blue Planet Metallic, Brown Seat, Forged Wheels
2014 Yamaha FZ-09