I am certainly not the Billet Box answer man as only a couple weeks ago
@BobsHere was helping me figure out how to get the boro/tank out
I can give some considered initial impressions now though.
The Billet Box concept is in a class by itself because it uniquely combines several elements of other designs into a single package.
Yes, AIO (kinda). Everything is contained inside the box except the drip tip.
Also yes, a modular design which allows a nearly endless array of setup possibilities from a significant number of drop in coils, some of them quite good, to several RBA bridges, which bridges also then each provide an array of possibilities themselves.
It can be run as a totally legit super tight mtl setup or a totally legit low to moderate power rdtl and anything in between.
ENDLESS aesthetic options. Door panels, internal cover panels, buttons, drip tips, a few different boro colors and even condensation plugs.
In other words, incredibly versatile, and in a compact rugged design, that while not tiny, is pocket friendly enough and portable. Also decent juice capacity, generally ranging from 3-5 mils which will depend on which bridge is in it at the moment. Also in that regard, once you get the seal on the boro lubed up and broken in, the best/easiest fill method ever imo. Slide the glass, huge opening, fill and slide the glass back up.
Next will be possible cons depending on the person. All of this is of course fully open to correction and revision from those with far more experience with the Billet Box than myself, which wouldn't take much lol.