Is it immoral to use a tool to aid in improving yourself? No, I don't believe it is. As long as the drugs do no harm, then there is only benefit to be gained though their use. However, if you become reliant on the drugs, they will harm you through your addiction to them, even if the addiction has no physiological basis.
Health risks are difficult to pin down.
This abstract seems to indicate that modafinil is safe to use over a 40 week period, yet 9% of the patients involved in the study dropped out because of 'adverse events'. So there's obviously some chance of side-effects.
And what happens if you use them for many years? What happens a decade or two down the track? I don't know if studies like that have been done yet, and until they have, I wouldn't call them safe.
That said, I wouldn't want to miss out on a great opportunity to enhance my self-improvement because of the possibility of it being unsafe in the long term. I believe life is here to be experienced, and I do try not to limit those experiences due to fear.