That's a bit of an exaggeration.
http://en.wikipedia.org/wiki/Modafinil#Pharmacology
Sure, there's no long-term studies, but we can extrapolate from the receptors that it acts on and its relatively minor side effects that it wouldn't be too bad to take it over a long term. At least for it's main indication, ADHD, it's far better than the alternatives of Ritalin or Adderall.