If everyone in the world had a unique ID, then projects like this would be a lot simpler. However that appears to be politically untenable, so far.
As it is, even in the UK alone, SRs are listed if they are AUK members (and so have an ID that AUK can use) but UK non-members are not listed. So in that sense even AUK's lists are not complete. (To be clear, AUK does retain records of non-member rides - from 1976 to the present - but only as individual records, they are not processed or summarised in any way.)
ACP's records are very complete (though I'm not sure to what extent the historical paper records have been databased) but they only pertain to BRM rides, so from AUK's perspective they are incomplete as we have many SRs and other results by virtue of BR rides.
Extending the AUK-style data model to a global scope is in theory pretty straightforward, but the obvious repository for such would be LRM, and over a very long period that 'organisation' has not shown itself to be a safe pair of hands, with officials of greatly varying competence/commitment, bug-ridden websites, etc. (No reflection on the current President but he has inherited a historical mess.) I considered pushing this project about 15 years ago but decided it would probably end up as a waste of time.