The main downside is that the effects are small - most of the research is in strength and endurance training and it seems like at most there's a 5-10% increase in lifting endurance (IIRC, it's been a while since I researched this).
It's one of a very few supplements that I personally feel should be generally taken by most people (unless you eat a lot of red meat).
The others are omega 3 (unless you eat fish several times a week) and possibly magnesium as the western diet tends to be low in that.
If you want to read more about effects of creatine on the brain this is a good paper:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7916590/
The consensus seems to be that supplementation of creatine is less clearly linked to increases in the brain than it is for muscle (although there have only been limited studies done), however, if you've ever suffered from depression, sleep deprivation or other neurological issues it's likely to be beneficial. For people without those issues, there's less evidence yet, but they would still get the benefit of more energy overall if not specifically more cognitive energy.