Hi Lindsey,
the sub queries are transformed to joins by the query transformation in cases where this makes sense.
Apparently, the query transformation makes quiet a deal out of your statement, since the current limitation for joins is
select * from m_system_limits
| CATEGORY | NAME | VALUE | TYPE | UNIT | COMMENT |
| DATABASE | MAXIMUM_NUMBER_OF_JOIN_TABLES_IN_SELECT_STMT | 255 | INTEGER | Maximum number of joined tables in an SQL statement or view |
So, re-writing the statement might be worthwhile to consider. 
- Lars