i just have a proposal to develop a "little" ERP system with Yii, but i want to know if there is any disadvantages to use Yii as a core in such complexe system.
I think it is save to say that one can’t answer this question without real facts about what you are planning to do. Yii is very efficient and fast so I doubt that your system will have any problems that are caused by Yii itself. The only thing that I would suggest when it comes to big data: ActiveRecord wasn’t made for reading and updating 1000’s of models at a time. It is fine for 90% of all use cases but it tends to be memory consuming and slower compared to simple DAO/PDO statements which is quite logical as AR is build on top of that. So my 2 cents: First go with ActiveRecord as it is easy to implement and already well tested and if you experience some performance bottlenecks add caching or revise your code and go with DAO. So to sum that up: Your bottleneck will (as always) be the database, not the framework itself