My old loft had a steel doorway that I used for pull-ups for two years straight... it taught me more about core engagement than any stable bar ever did. rose_martin22 mentioned the limited range, but that forced me to control the eccentric phase like nothing else. Actually improved my bar pull-ups because I had to stabilize every millimeter... the instability wasn't a bug, it was the feature. Felt way more prepared for outdoor rigs with uneven bars.
That instability as the feature point is SPOT on. It trains your body for real world chaos, not just gym perfection. Suddenly a stable bar feels like cheating.
Hmm, doorway sessions are great, but doesn't that limited range make it harder? A real pull-up on a stable bar must have felt like a whole new victory.