The favor that Jackie Chan owed to Jimmy Wang Yu was that Wang Yu had negotiated on Chan's behalf during a Triad backed dispute over his contract between Golden Harvest and Chan's former employer Wei Lo. Wang Yu used this debt to not only get Chan to make this movie but Island of Fire (1990) as well.
Featured in Rob Hill's The Bad Movie Bible.
When the Fantasy Mission Force is in the village of the Amazon women, some of the music is "borrowed" from the La Planète des singes (1968).