The “killer feature” term might have originally been coined by the Duplicacy developer in their forum:
pointing back to the Duplicati forum:
Both articles are a little dated now. Duplicacy has now gone over to web-based UI like Duplicati uses, however it’s licensed and extra-cost. The good/bad thing about that is it might be allowing the original Duplicacy developer to remain actively involved, whereas Duplicati might be in some transition mode.
Duplicati probably still wins on feature quantity, but has trouble with scaling and stability (i.e. it’s beta). Possibly its web UI is still better than Duplicacy’s new paid one (which I have not tested). Trade-offs… There have been Duplicati users leaving for Duplicacy. You can search this forum and theirs for notes.