OnePlus One smartphone was the first flagship killer. This super powerful mobile with Qualcomm Snapdragon 801 chipset was released on the market over two years ago and it has Android 4.4.2 (KitKat) on board. Officially, OnePlus One is upgradable up to Android 6.0.1 Marshmallow. The official reason of the lack of Android Nougat update for this smartphone is lack of graphics drivers from Qualcomm.
A lot of people blaim Qualcomm for this, however they do not release Android Nougat drivers for Snapdragon 800 and 801, because these chipsets do not support Vulkan API which requires graphic unit with OpenGL ES 3.1 or 4.1. However, XDA-Developers are very capable and they have ported Android Nougat to OnePlus One smartphone.
This Android 7 Nougat port for OnePlus One is very decent.
Very often customized ROMs have some limitations. I have seen a lot of ROMs with not working WiFi or GPS. Some of them have also issues with camera etc. However, this Android Nougat port for OnePlus One is very good, because the only limitation is lack of video hardware encoding, so videos can not be recorded properly. Also, GPS module needs more time to determine the initial location, but it works. As you can see this is very interesting that people from XDA-Developers forum are able to create Android Nougat ROM even if they do not have graphics drivers from Qualcomm.