When i first installed 10 on my 640XL it was flawless too. But as time passes by, problems starts springing up that only hard resets can fix, so keep your fingers crossed :X
Hmmm i would not really recommend it as a Lumia 950XL is pretty expensive as it is a flagship. Well the hardware may not be a problem, the software is what i worry the most as Windows 10 mobile is pretty in a bad shape for me right now