If you upload an ADIF file and there is a problem in your log (usually a bad REF), you will get an error message that is not very helpful in locating the problem. If you upload a CSV file and there is an error, you will get a very useful summary that shows you exactly what the problems are.
I know that Andrew is working toward making the ADIF processing just as robust as the CSV file processing, but we all have day jobs and limited time for SOTA IT work =)
Bottom line: if you get an error when uploading ADIF, convert to CSV and upload again to see what the problem is.
-Josh WU7H