Any problem will all InnoDB and one MyISAM table?
Is there a maintenance or tuning headache here? ABSOLUTELY !!!

Any benefit to it? I'll pretend I didn't read that.

The MyISAM storage engine always performs a full table lock with each DML (INSERT, UPDATE, DELETE). SELECTs block the daylights out of DML Statements.

Any transactions that mix MyISAM and InnoDB tables will cause even InnoDB tables to degenerate to full table locks because of the presence of even just one MyISAM table. I wrote about that in StackOverflow back in March 2011.

If the only MyISAM table is just the log table you mentioned, that is still very bad. The reason? If 50 DB Connections want to write a single row into the log table, there will 50 full tables locks, 50 inserts. Here is what is worse:


50 DB Connections, 1 performs table lock, does INSERT, 49 DB Connections have to wait
49 DB Connections, 1 performs table lock, does INSERT, 48 DB Connections have to wait
48 DB Connections, 1 performs table lock, does INSERT, 47 DB Connections have to wait
...
1 DB Connection, 1 performs table lock, does INSERT


Get the idea ??? This is what awaits you by making the log table MyISAM !!!

If the same log table was InnoDB, 50 DB Connections would perform 50 one line transactions that do not block each other.

Please read these questions for your own benefit on MyISAM and InnoDB


What are the main differences between InnoDB and MyISAM?
Benefits of table level locking
Should I use a storage engine other than MyISAM to optimise these tables or should I get better disks?
Is it common practice to mix InnoDB and MyISAM tables on same server?


CAVEAT : To be fair, if a MyISAM has a FULLTEXT, then you have no choice but to live with it. MySQL 5.6 will change that problem as InnoDB will support FULLTEXT indexes.