There's no doubt in my mind that
unfulfilling should be accepted in future. The only question is: rare or common?
The word was, oddly, little used until the tail end of the 20th century, which probably explains why it was not accepted previously. (Although words of over 10 letters were added to the list only recently, for use in 7-by-many puzzles, they were derived from the same word list I used for the original words, back in 2005. That list was then a few years old, and had been based on dictionaries published some years earlier, which reflected word usage from some time before that...)
The OED has not fully updated its entry for
unfulfilling since it first appeared in 1924. It gives one citation, from Shelley: "Alas! for Liberty! If numbers, wealth, or unfulfilling years, Or fate, can quell the free." The OED assigns the word to frequency band 3, explained as follows:
These words are not commonly found in general text types like novels and newspapers, but at the same they are not overly opaque or obscure. Nouns include ebullition and merengue, and examples of adjectives are amortizable, prelapsarian, contumacious, agglutinative, quantized, argentiferous...
I'm willing to go out on a limb and say that
unfulfilling is quite a bit better known than any of those words. Furthermore, the word
is now quite often found in novels ("Lauren's life has been so much like a short, unfulfilling magic show, full of failed tricks and disappointing illusions." -
The Christmas Town, 2017, by Donna VanLiere) and newspapers ("An absolute slog of a summer would seem to point to an unfulfilling end for manager Joe Maddon's club." -
Chicago Sun-Times, September 2019).
Recently my views about which words are common have been influenced by a word list called 3of6game, which seems to match fairly closely my ideas of common words. This list includes the word
unfulfilling, but with a notation identifying it as a "signature word". These are words that the compiler of the list, Alan Beale, believes should be included even though they don't meet the relevant criteria. So presumably
unfulfilling was in fewer than 3 of the dictionaries used in compiling the 3of6game list. But perhaps that's because the word's usage has been increasing over recent years, and not every dictionary has caught up with it.
Or maybe some dictionaries feel there's no need for an entry for the word since
fulfilling is listed, and
unfulfilling just means "not fulfilling".
In any case, I'm adding
unfulfilling to our list as a common word.