While Modafinil is much safer than most other stims it is an inducer of CYP450 3A4 which means that it reduces the efficacy of other drugs like Oxycodone, Hydrocodone, Benzodiazepines, etc. It also has effects on hormonal birth control, even weeks after discontinuation. But as for safety alone the only real issues are some skin reactions and hypersensitivity to modafinal itself, both of which would present very quickly after a test dose. The US Air Force has switched over to it from DexAmp and the abuse liability is considered non existent at normal doses.
The mechanism of action is kinda interesting though as it works at the same site as cocaine, though in a different manner.
https://www.ncbi.nlm.nih.gov/pubmed/23933217