Do you think underwriting even deserves its own term? Or is it just a structural/planning/pacing deficiency? Or perhaps you think that every story has its own natural length that should be embraced?
A few months ago I watch Princess Mononoke for the first time. For a myriad of different reasons it took me many many years to watch this masterpiece. Watching it, made me look more into Hayao Miyasaki and how he came up with the story and concept and how he incorporated his imagery to this film. When I learned that he personally amended many of the cels used to make this film I realized the dedication and skill required to create something spectacular.
One thing that I didn't know was that his studio; Studio Ghibli, pitched it to Hervey Weinstein to distribute it across American and the Western world, and Weinstein ordered Miyasaki to cut the 2hr 13 minute run time to 90mins. Miyasaki refused and sent Weinstein a sword to indicate 'NO CUTS.' It worked and the film was left to its original length and how Miyasaki imaged it.
For me, there are cultural differences in story telling between the East and West (and sorry for this long winded answer to your questions) but I don't think there should be a target to tell a story... brilliantly. Cutting is needed to make it more concise etc... but the length of the story, how a writer plans and structure in revealing the story to a reader should feel natural and if the story climaxes 20k short of the 'target' then it finishes 20k short of the target.
I prescribe to the view that if you change one thing (note change and not add) it affects everything after.
Removing a word or a line can alter the ambience and flow to a passage and the same principle goes to adding. Add something and it can push the narrative towards another direction and you lose that core message you wanted to give the reader. Is this a good thing? Well if it is to reach a number then no. If adding and altering the message actually gives you and a reader a happier outcome then great... but you should not add for the sake of a number.
The structure, flow and messaging of the story is very important to me and this can be lost through needless addition. I would only classify a story as being underwritten if (let's say) a main character stands with our hero and there is little to no detail about them as they act as a voice to the protagonist. It can be said that under developing a character is a deliberate choice but, again personally, if something is left undeveloped or making me ask for more for clarity, then this is underwritten.
If I am reading about a Fantasy world and I am struggling to form a picture of this world... that is underwritten, but if feedback from a reader points to the story reading well etc... and the only issue is that it missed a word target... then it misses the word target. A reader's reaction is far more important than keeping to a word count.