There may or may not be a downside depending on what one considers a downside.
In one of my other comments I just made in this thread, I mentioned my experience taking relatively large doses of 20g a day. While I found it has cognitive benefits, it did interfere with my sleep, though not catastrophically. If a person happens to enjoy sleep, then it's probably best they stick to 5 or 10 grams. On the other hand, if you need to pull an all-nighter, the sleep interference (as well as the better recovery the following day) may not be seen as a downside but beneficial.
But yeah, from a toxicological perspective, creatine does seem very safe even at those doses.