A problem I personally have with such roleplays is that I like to have them be character driven and exploring the human side of war rather than just being constant action(of course some action is inevitable since it's, you know, war). However, many people who participate in these seem to just be interested in the action and combat aspects of it and showing off who can be the most badass Rambo character. I'm not a veteran myself, but plenty of veterans can testify that there's more to military life than just fighting on the front lines. That life behind the front lines is something I like to see explored as well that not too many roleplayers seem to be into. So yes, I think a lack of audience tends to be a big problem with trying to establish a military RP.

For the record, I've been craving a WW II character driven RP.