A user on the OnePlus forum has mentioned that his OnePlus 5T is unable to stream HD content from popular streaming sites like Netflix, Amazon Prime, Google Play Movies and more. This issue revolves around the lack of support for Widevine Level 1 DRM which is required to stream HD content.
Currently, the 5T supports Widevine Level 3 which only allows for SD streaming on the device. This problem is also seen on OnePlus 5 which shares the same Widevine level as its successor. OnePlus has said that there will be an update coming to enable Widevine Level 1 supports but did not say why the phone did not come with the support or when the update when be rolling out. Most of you already know the specs that the 5T carries, and based on that can easily make out that the phone is more than capable to support HD and even 4K content easily.
However, streaming YouTube videos in HD seems to be not an issue on these phones, but if you have Netflix or other video streaming services you might have to wait for a while more before you can enjoy Full HD resolutions on the OnePlus phones.