ADOdb only supports features native to the underlying DBMS in use - otherwise it will return a false on the function call or more likely ignore the query call, or return the SQL passed. Same goes for prepared statements, and other such advanced features.
It is indeed very complex, to my mind its far too complex. There is a lighter alternative called
ADOdb-Lite which is a lot simpler, does much the same thing, but supports fewer DBMS's (not that it matters since generic SQL is only useful on a subset...).
Also be aware ADOdb's power is not only a generic API. It can be used to generate or at least choose from a selection of SQL commands specific to databases. PDO for PHP5 is often quoted as being a solution to abstraction layers but that's not totally true.
ADOdb also packages a lot of extra functionality - DateTime functions, PEAR compatibility functions, Meta functions (for collating meta information on tables, rows and the database itself), caching (intelligent caches native to DBMS are superior), Data Dictionary (representing a database structure as XML which can be translating into DBMS specific SQL for table creation or difference updates), and more besides... The complexity is that many of these are not always separate from the main class - ADOdb is not a modular library.
If you need more simplicity, ADOdb-Lite probably is easier to read and follow. I've been using it a lot recently and I've been
modifying it to add a Performance Management log and monitor similar to that from ADOdb. Maybe it's more open to modification and ease of understanding since it's a lot smaller and uses a Module system for including extra features which ADODB just dumps into the main classes?