In my experiences, kratom can be a surprisingly powerful opiate if you buy the right strains and consume the right amount. It almost certainly will be extract if you want it to even touch the potency of pills or H. I remember one summer I spent away from home after the very worst of my heroin addiction. It was absolute hell for the first few weeks of course, but once I discovered kratom extract from a local head shop, I found it to produce surprisingly satisfying opiate like effects - especially compared to the new normal for me in which I had tylenol 1 (8 mg codeine with tons of caffeine) at best. It can certainly become addictive, however I noticed two properties unique to the herb (at least in my experience). Although it undeniably affects opiate receptors in some form (classic feeling, miosis etc.) it does not seem to be fully cross-tolerant. If you are coming off a severe heroin addiction, even copious amounts of potent extract don't always seem to completely alleviate withdrawal. It certainly helps, but not like you would expect something like methadone would, for example.
Another thing is that even heavy usage does not seem to produce withdrawal anywhere near as severe as pills. I suppose this makes sense since it takes longer to act and the effects are generally more mild. I've heard that you can find incredibly concentrated UEI online which can actually compare in potency to heroin, but with many warnings that withdrawal from this absolutely can be as severe as H if not more so with longer duration. You might as well stick to pills or H if you're going to get yourself heavily addicted to kratom, although in general I definitely see it as a valuable way to taper down your usage and still achieve that magical opiate high after quitting the strong stuff - the hardest part of quitting for me (after physical W/D and PAWS of course) was the thought of never again experiencing that opiate feeling. Kratom seems to allow this with minimal addictive potential if used sparingly and properly. In fact, after not using any opiate for weeks to months, kratom gave me a definite shock in that I didn't think I would get that good of a feeling from it the first time. It was actually somewhat comparable to the first time I used poppy pod tea albeit less intense and addictive.