disclaimer: these are just my own views at this point in time.
Judging from my own experience, and what I have read from others that have taken sub-flood doses, you can have amazing healing experiences, saying that others have had hellish experiences. If you are serious about this medicine and feel called to it, then a flood dose is what you should do taking it first time. My breakthrough experience was my first flood, everything was just 'right', the time I took it, where I was in my life etc... Think a lot of it rests on your relationship with the plant alley, your respect for it, but not only that, its also how the plant alley gets on with the person taking it, which IMHO, is the calling.
Just taking it because it sounds cool is extremely risky and I am with Kampum, these allies work on the spiritual/subtle energies as well as physical. Perhaps lower doses meeting deep issues and lack or respect means iboga will not be there to protect you enough? I've always felt reassurance from Iboga, but I have also felt its wrath when I have taken it without preparation or for selfish means. Yet, it was always there. My 2 times taking an ayahasuca analogue I felt far more aware of and exposed to distinct entities, but this could be due to my cavalierness and lack of respect in hindsight.
From the animistic point of view of the pygmies and bwiti, and other cultures based in shamanism, everything has spirit/spiritual energy and everything is connected, thoughts can take an energy or life of their own in these realms (sometimes called thought forms or egregores). When seen in these realms Negativity/trauma can manifest as dark entities (like a coiled snake), or blockages connected to someone. Then there are distinct spiritual entities themselves that can interact, attach or possess.
When healers journey (via plant alley or other means), the shamanic practitioner did for me, they become a 'hollow bone' for benevolent spiritual entities which then through the shaman, interact and remove the blockages/entities that came from past trauma or negativity/fear or unhealthy attachments/beliefs. Part of the process was then telling me what was removed and why it was there, along with a message from spirit, in the form of an affirmation or prayer.
Similarly iboga, IMO, allows the taker to be a 'hollow bone'. It got me to see my own issues clearly, and to meet them head on, and so the energetic purge, be it peeing, throwing up, shaking and crying to me is a release of psychic energy, and generally part of the healing. Its like a transformer where the negative thoughtform/blockage gets downgraded to be expelled or something. Perhaps this is iboga pushing it out? One thing I have learned with iboga is having a clear intent or request going in WILL get answered in some form. In my journal the 'archbishop' entity I saw after a purge was a release of my deep beliefs I had harbored due to dogma learned in childhood. Since then my spiritual pursuits are not mired in that guilt that was holding me back. So in that way, I have seen commonalities in my journeying with iboga and that with drum & rattle, intent, ritual and respect is absolutely key. You may not 'get' or like what you see either! As Michael Harner said, they are just different ways to access the same realms.