In short: See what xlLens reads directly and which workbook features are not implemented yet.
Supported
XLSX and XLSM use the streaming workbook index. XLS and XLSB use a bounded compatibility path. CSV and TSV use streaming reads, including UTF-8 and CP949 decoding.
Current limits
Password-protected workbooks, 3-way comparison and merge, and chart or shape visual diffs are not implemented. xlLens fails clearly instead of presenting partial results as complete.