From my understanding weaker camera support in nexus is partly due to google providing binaries (drivers) along with documentation. Companies don't want to give up their software special sauce through the open source requirements that come with nexus. In other words you can count on the camera not being as good as the lg G4 I would wager if the sensors are the same, you know I don't keep up with that
. Whether this is due to the drivers themselves or the lack of lgs camera app is over my head.
The GPE program showed us that GPE devices had weaker cameras as well and that program did not require release of binaries like nexus. So it seems there is a stock android tax and a nexus tax
But.. I believe nexus 5 is the oldest phone with camera2 api, so you can count on future api support where other oems probably won't follow through.