The Gorilla Glass 5 is known for its very durable drop protection and unfortunately, the rear glass feature Gorilla Glass 3, not the latest Gorilla Glass 5 like used in the front and unlike the Samsung Galaxy S8 that comes with GG5 on both the rear and front. On some Galaxy S8 durability tests the S8 did disappoint. I wonder if this is because of the 3D glass and whether there will be a different between the S8 and the U 11 in that regard.
That being said, Gorilla Glass 5 is the best hardened glass we have right now I believe. So if the GG5 is likely to break when dropped even from waist height, the Gorilla Glass 3 is even more fragile than that. I personally prefer putting a case on every phone I have. The thing is that the HTC U 11 rear looks so nice with its reflection effect and I wonder if putting even a transparent case would make this effect less impressive, I believe so.
If I buy the HTC U11, I'm sure that I won't count on the GG3 to protect the back in case it falls on a hard surface. The back is glass not metal, so it might break and this means that I will need to send it to replacement. I don't think replacing this beautiful rear part is going to be cheap. If there was just a plastic piece or not any sophisticated design there it would probably not cost too much, but now with that unique layers glass design, I am kind of worried.
I first thought that both the rear and the front are Corning Gorilla Glass 5, but according to the description in this video by MrMobile, and you can read it yourself, HTC has confirmed to him that the backplate of the HTC U11 is Gorilla Glass 3, not version 5 has he mentioned in his video (nice to put a correction for it). I also went to check this information on htc.com, and on U11 product page in the technical specifications' section, it only say "3D Corning Gorillas Glass 5" in the display section, but there isn't any mention of GG3 anywhere. So just so you know that the rear part isn't GG5, so I think it's better to make sure you get some sort of a protective cover just in case, I'm sure many of you don't want to deal with that rear glass breaking and stuff.
I think that for a flagship device like the HTC U 11, HTC should have gone all-in and use GG5 for the rear panel as well, especially considering that to showoff the phone in its full beauty, some users might prefer using it without a case at all. So the phone has to be as durable as it can be considering its price tag.