As you point out, the deuterium fusion limit has a lot of variables. There's problems with using it as a cut-off between the planet-BD bondary as young, higher mass gas giants are able to fuse deuterium for a brief time before they cool down. As a result, some of the objects that we currently call planets may well have been fusing deuterium in their past.
A better discriminate may be the method of formation. Brown dwarfs may form by disk instability, and planets may form by core accretion. A problem with this, however, is that some objects we would rather consider brown dwarfs are in neat, orderly orbits, often with multiple other objects (Ups And, HR 8799?, Iot Aur).
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