"This way, there can be one, linear, version of historical events that dont depend on interpretation, "
RL Example: In WW2, During Battle of Britain both sides were convinced that RAF was on verge of collapse. This belief shaped strategical decisions - RAF even started saving aircrafts, in order to still have some reserve, which made Germans think that not much is left. The only problem... recent studies show that both sides incorrectly estimated other side production rate and in the darkest hour Allies were decisively winning aerial war of attrition.
This information may be hard to get from a collective consciousness, but a dispassionate quartermaster (or accountant) may be a bit more useful. ;)
You don't want to make it a game breaking? Except already raised issues like false memories (including hallucinations), emotions... Why the process should be immediate? I mean if a soul needs a few decades to truly merge, then it does not matter for ancient history but makes it mostly useless from military perspective.
There may be even some interesting mechanism that for example sinner, individualist or very stron personalities would simply need a much more time to merge.
EDIT: Events equivalent to... evolving on savanna? Almost getting extinct 70k years ago? Leaving Africa? Ice age cycles? Slaughtering all megafauna on the way? Having sex with other hominids like Neanderthals and Denisovans? Domesticating plants and animals? Going in huge migrations throughout continents? (recent DNA test show that Europeans are mixture of local population + Middle Eastern + Siberian)
It can be quite interesting as history would start at dawn of mankind and not at invention of writing.
