Chance of skipping extended events when using an offset for the file target
I couldn't find a definitive answer about how the offset is generated, only one unreliable reference that they're a block number. A little disappointing, I guess the engineers don't use stack exchange. 

However I was listening to the SQL Down Under podcast this week and they were discussing the issues with doing ETL based on timestamps which are a little unreliable and result in skipped records. 

The advice was that it's a good practice to read 30s back from where you left off and then eliminate duplicates. 

So unless someone comes up with something better you can at least address your missed record concerns by starting your reads one block prior to the last block you received, and then eliminating duplicates.