I do not see some columns in the mirror database such as incident comments.

SnowMirror stores the data the same way as they are stored in the ServiceNow database. Not the way the data is displayed in the ServiceNow web application. So for example the journal fields are not stored directly in the original table (e.g. incident) but in a special system table called sys_journal_field. To view incidents together with comments you need to synchronize two tables (incident and sys_journal_field) and then perform a join to display the data together.

The same applies for referenced records. The web user interface and even some exports (e.g. ODBC or exported XML) contain a display value of a referenced record (e.g. incident caller name). However SnowMirror downloads only a sys_id into the caller_id field. And again to obtain all the data you need to download two tables (incident and sys_user) and join them together.