Request too Large / This action would increase the number of cells in the workbook above the limit
If your jobs are failing with a "Request too Large" error message, it means you are violating Google's cell limit which it imposes specifically on the Google Sheets format (distinct from the CSV format in Google Drive, to which no limit applies).
The solution is to flatten to CSV (not Sheets) in Google Drive, or try a different cloud storage destination altogether (OneDrive for Business, etc.)
The failure might also occur due to too many columns in your flattened output -which is also restricted by Google. Flatly is working to maximize rows over columns to resolve this problem.