JD will use the host link checker (usually).
If there's an issue with the hosts link checker then JD will have the same issue.
Fileserve:
http://board.jdownloader.org/showthread.php?t=35568
Also, it appears RS is/was blocking free downloaders (specifically JD users).
Rapidshare:
http://board.jdownloader.org/showthread.php?t=35569
JD program uses your own IP to connect to filehost links to download.......and it uses host linkcheckers to check links.
Any issues (
afaik) will be related to IP country blocks (if applicable to you), filehost issues and link checkers of filehosts.
Check the JD forums for info about any issue(s).
http://board.jdownloader.org/?langid=1