I only know that if you're not careful with kratom it will hurt you thru the exact same mechanisms that opiates do. No idea about dosing or anything on kava or kanna. I tried kanna but only low micro doses. It is like a slight mood elevator at low dosages, like I imagine "a beer with dinner" elevates mood slightly.
2.5 grams of kratom feels exactly like an oxy, down to wanting to tell everyone you really love them and is that shirt felt well it is now