Two problems I have found in regards to editing description.ext

1. overviewText seems to override onLoadMission text. So in effect if you want overviewText then you won't be able to have a different, shorter description for your loading screen as it will just be the same as the overviewText. The same doesn't apply to the picture.
2. overviewText seems to be limited to a low number of characters - far less than would fill the overview box even if on large UI. Anything over a certain character count just gets truncated.