Hi there David,
Mass Updating of any sort is typically faster using the Import / Export tool (unless its something you intend to do often) - Would you mind sharing some of the Bugs you ran into when trying to do this?
If you would like to follow the Customization route - there are a few considerations before pulling the trigger on a lopping process. Most importantly - are all the records in the table you are looping through being assigned to one person? Its not a problem if they are, but you will need extra logic to determine who to assign a record to otherwise.
If you could send a screenshot of the action set you are using, we can take a look at this together.
Thanks David