Wikipedia has a summary of the changes:
In 2012, a "Technical Update" was applied to the N63, resulting in the N63TU variants (also known as N63B44O1). The main upgrade was the addition of Valvetronic.[5][6] Other changes include revised turbochargers, removal of the blowoff valve, lighter pistons, forged connecting rods and crankshaft, addition of a valve cover labyrinth oil catch/return system, new valve stem seals, revised fuel system and addition of a second coolant pump.[4]
When I was researching N63t (N63TU) 2 years ago I didn't find any failed engine threads, while you won't have to try hard to find them for N63. Valve seals also appear not to be an issue and seem to last for 100k+, while N63 owners were having to replace them every 30 minutes. You also get a nice performance increase
. Slow oil consumption overtime is an issue with the N63TU, from what I've read most owners need to add 1 quart every 2-4k miles (myself included).
I bought my 2014 550 about 2 years ago which has the N63TU, and after researching at the time I did not have any concerns with owning the car, and still do not today now at 75,000 miles. So far it's only needed standard maintenance: annual oil changes, brake fluid change, and spark plugs at 60k miles. Many people will mistakingly categorize F10 550s with N63/N63TU together, so be sure to dig deeper when you see a "550s are bad" thread. Of course, do your own research and to come to own conclusion because ultimately you are the one who needs to be comfortable owning the vehicle.
Also since you also mentioned X-Drive, mine is RWD as I didn't want X-Drive. I'm not sure if X-Drive has any concerns/issues but you might want to read up on that topic as well. Good luck