This was a very enjoyable read. I built a nearly (architecturally) identical system a few years back that also had to be scrapped for different reasons. This brought back a lot of memories. The sanity checks, the index reconstitution issues, dealing with the insanity of security identification and tracking through time.

The fun cases are the ones where it's not even clear what the right answer truly is, e.g. company A spins out company B, and then 5 years later they re-merge. Who's time series and associated data is "the" canonical one? The data vendors often try to give their answers to this question, but maybe their answers don't make sense for your analysis.

Then there's the fact that a lot of vendors don't really do point in time correctly. They like to go back and helpfully revise data points for you that they or the company initially misreported. This is all well and good except that if you were trading for real, you wouldn't have known the correct information at the time, and so any backtest based on the updated information will be invalid. Vendors are a bit better now about providing true point in time data sets, or at the very least accurately describing when they are/aren't doing this. But we had a few cases where they said they were, but they definitely weren't.