I've got the same phone and have had little to no issues with W10M on it. After a week of use, my battery life has settled down significantly. At first I would burn through 70-80% charge while at work. Now I'm barely using 30% and that's with playing music through a bluetooth speaker all day.
Most of the issues that I initially had all went away after the first hard reset. I was on the .29 build and hadn't reset yet because I wanted to look around and see what the new OS had to offer. I stumbled a few times during the first day with strange behavior from the phone. After reading up a bit more here on the forums, I went ahead and backed up my data again, took a few screen shots of my layout so I could recreate it, then did a hard reset. Since then, everything has been great. I updated again to the .36 package, and it's been running fine without any other hard resets.
So, to answer your question: I personally think that it is safe to upgrade a 640LTE provided you backup all your data first. Hard reset the phone at Win8.1, then install the update to .29 or .36 (whichever is offered to you first) then do another hard reset before installing any apps.