as the vapour resistance is greater, this does not mean the air tightness is reduced. good vapour resistance can be achieved in a number of ways, parge coat on blockwork, polythene sheet on timber kit or as i'm doing https://mdfosb.com/en/smartply/products/smartply-propassiv these prevent the vapour from entering the insulated area of the wall. on the outer skin of the kit i have https://mdfosb.com/en/medite/products/medite-vent which is vapour open, this allows for vapour to leave the kit, effectively allows the wall to breathe. the heat is kept in by the cellulose insulation and as the smartply is vapour resistant and all joints are glued and sealed there are no draughts, as it provides the air tight layer as well and therefore little loss in heat.
if the kit is built quickly there should be very little movement in the i beams due to the majority being osb and a 350mm i beam isn't going to flex much, so i wouldn't think very much less stable than concrete. it has been observed that a good deep layer of cellulose is eerily quite inside a house due to being pumped to a compaction of 55kg/m3.
as for negative effect on landfill, you are looking at a natural material apart from the formaldehyde glue which would decompose over time as opposed to concrete which has a very high co2 impact in its manufacture and will not break down though could be recycled if it didn't have another potentially non recycleable product such as polystyrene attached to it. the environmental impact of production and end of life materials are not good as opposed to timber and paper. as to costs, i haven't compared to concrete.
partially controlled draughts
what is internal insulation? and all houses will overheat depending on design. insulation is there to stop the heat entering or leaving the building. in most instances in uk, leaving, though down south it is becoming equally important to stop it entering. if you have a house with sooper dooper levels of insulation which has a high decrement delay, this can all be wasted if you have huge amounts of normal 3g glazing to admire the south and west views as the house would potentially overheat due to solar gain.
i'm sorry, you need to do a lot more research before asking questions like that.