From your link "Kratom contains over 50 alkaloids—primarily mitragynine and 7-hydroxymitragynine—which act as partial agonists at μ-opioid receptors"

The addiction is the kicker there. Kratom withdrawals/dependency isn't as bad as traditional opioids/opiates. But it's a magnitude of an order worse than nicotine/caffeine/etc. dependency and withdrawals for some people. And the full safety profile of people taking concentrated kratom extract for long periods of time isn't really known yet.

I'm really of the belief that the only reason kratom exploded in popularity the way it did was because it was a legal alternative to opiates. Not necessarily because it's an upgrade (or even sidegrade. ) If morphine could still be purchased in every corner store and pharmacy over the counter kratom extracts would never have blown up the way they did.

Johns Hopkins: "Kratom (Mitragyna speciosa): Pharmacology and use of a naturally occurring atypical opioid."