I did a refseek search, because I hadn't heard of this alternative. It lead me to a few references, several of which contain this quote:
The United States Drug Enforcement Administration state "There is no legitimate medical use for kratom".[13] Kratom has become popular as a recreational drug and has been promoted with claims that it can improve mood, relieve pain and help with opiate addiction.[9]
(That one's from the Wikipedia article)
There's a lot of concern because of the opioids/opioid-like substances within the plant, and how people are misusing it, and not weaning off properly. Also, I think, the grade of the products they purchase.
Personally (because I'm sensitive to a lot of medication) I'd avoid it.