The Nightly, Testing, Unstable and Skinny are Developer builds.
They in no way reflect which area of Devuan these are built from.

Nightly => Nightly builds by `cron` available to testers. The last place we test changes (from test builds) before being considered for a Release
Testing => A place for ideas that may or may not ever make it into the Nightly build(s).
Unstable => Neurotically broken area where things are put there because they are maladjusted. This is where attempts to get them working are done.
Skinny => An effort to get an even smaller ISO while still retaining the original functionality. It's trim and it's sleek. It might even boot to a CLI as we are working with it.
You have to break some eggs to make an omelette.
These areas are where various eggs get broken.

There is nothing special about any of them.
In the boot menus, there is a reference to Developers Desktop pointing to /dev/sda126 .
This is so I can leave the USB stick in my Dev Desktop and boot to a partition on my HDD instead of into the live session.
It saves me a lot of time, when doing trials on real hardware.

The apt-pinning:
Standard pinning for installed components is 100.
IIRC , the backports area has the same pinning.
The testing branch, we have pinned with a priority of 50 .
And the unstable branch, we have it pinned with a priority of 10.

This keeps new users from shooting themselves in the foot, as easily, when adding testing and unstable repos to /etc/apt/sources.list .
They can add them to their repo list or the repo list directory but nothing gets installed from there , by default, until the pinning file is removed or edited.
We get a lot of users , new to Linux. This one small thing (apt pinning) saves a considerable amount of "hand holding".

To the best of my knowledge we have never intended to build from the testing or unstable branch.
In my opinion, we'll leave that realm for the developers of other distros to pursue.
We want it to work and not be a "toy" requiring constant fiddling. If their approach is working for them , Fantastic.
I want a fast car too , but I'm not adjusting the lash on solid camshaft lifters every weekend to get it.
And I'd rather put my small amount of time available, towards something besides tinkering (with something that wasn't broken) and support.

Anyone that wants to change up to testing or unstable is welcomed to do so.
If there ever were a warranty of any kind (and there isn't) this would most certainly break that warranty.