Here's how you can check your phone is TLC or MLC, install "IOKitBrowser" from here: (no jailbreak required so this doesn't against the apple's rule).
http://www.pgyer.com/IOKitBrowser
after install search for "device characteristics", and keep tapping the result until you see something like this:

Look for "default bits per cell", 2 means you got a MLC phone and you are fine, 3 means you got a TLC phone so you may probably experience some random reboot problem and keyboard missing etc issues. That really depends, according to my test, I have the random reboot problem when I used around 50% (around 600+ apps) of 128Gb (on two iPhone 6+s), and have keyboard issue, missing character issues and bunch of issues after I installed around 800+ apps. (10-20Gb left and crashes on both two 6+). I can say it's a very general issues since I only got two TLC phone, and haven't tested MLC iPhones yet, but it is possible that all the TLC have this risk till you install some amount of apps or used some amount of storage.
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Announcement for today:
OK, I just get back to the store again, and they give another replacement for free, but I have to wait till my new iPhone send from somewhere else to the store. I guess I may going to receive a new iPhone with MLC instead of TLC, since apple has already banned the TLC this November. Guys the only way to solve this problem is bringing your iPhone right into the store and ask for help ASAP, those people from apple store are really nice guys, I can tell they really want to solve the problem as urgent as you do, and also the only way to send those problem iPhones to Apple.