AWS RDS MyISAM tables from mysql and information_schema and good snapshots
The MyISAM tables in the mysql schema are fully supported in RDS snapshots.  It's the MyISAM tables elsewhere that are potentially problematic.

Never alter the tables in the mysql schema.  That will leave you with a server that may never start up again.


  Important
  
  Do not convert MySQL system tables in the mysql database from MyISAM to the InnoDB type. This is an unsupported operation.
  
  https://dev.mysql.com/doc/refman/5.7/en/converting-tables-to-innodb.html


The exception for the tables in mysql is because RDS takes care to flush and lock those during a snapshot, so they come out okay on the other side.  This isn't documented but can be observed.

The tables in information_schema are not real tables.  Leave those alone as well.

The data in MEMORY tables is transient, and not persisted to disk, so they won't survive a snapshot or even a restart.  Their definitions should be okay but RDS is really intended for use only with InnoDB.  (The FEDERATED storage engine is disabled.)