I see all the other two-word Apache repositories use hyphens not underscores, and on GitHub they are URLs which as has been pointed out should ideally be hyphenated? To me the underscore looks wrong...but, in the grand scheme of things, it's just a name and, like Sebastien, I will not lose sleep. I lose enough of that already, getting trying to get NuttX working on the processor I'm using LOL.
There are some toolchains supported by NuttX that cannot handle hyphens in paths. So, setting aside personal preferences, there are valid reasons why hyphens should be avoided. There are also old, obsolete filesystems that do not support hyphens in names.
Some file naming standards, however, do recommend hyphens over underscores. The primary reason seems to be that if the file name is underlined, then the underscore character is mostly occluded.
Paths may not begin with hyphens in most commands because the path is interpreted as a command option.
These directory names are never really used in the build, however; the name $TOPDIR which defaults to $PWD/.. when the OS is configured. The user selects the actual name of the directories when repositories are cloned so, regardless of what is decided, it need not effect your way of doing things other than the git clone.
For example, in order to build with those troublesome toolchains mentioned above, I have always cloned the repositories to incubator_nuttx and and incubator_nuttx_apps to avoid the hyphens which do cause build failures with those toolchains.
