Create API to handle data from multiple devices. Consider there is a app with 3 buttons 'Create', 'Update', 'View'.
Sync data between devices if there is any update on server side or within one device than it should sync with other device also.
Create scenario where three to four tables are involved and depend on each other. i.e. data is fetched from one table and then based on that some data should be inserted in second table and on the basis of result from second table third table should be updated.
Handle each possible case of failure or best practice to optimize the process.
You mentioned about devices in your project description. And I didn't get the picture of what you meant.
Anyways, as far as Yii is concerned, I have rich experience in working with YII