The Microsoft JET engine database driver for Excel and Access has a limitation of 255 character, this is why only part of your comments are being displayed when you connect Tableau to your Excel file.
If your source data sits within a database then you shouldnt have this problem and you'd be better connecting directly to the source. If the data only exists in excel then there is a workaround wherby you can split the comments in excel and then concatenate them after import and tableau will then display all characters.
This thread may help you (there are some others of a similar nature as well)
hope this helps