Limitations

How much data can we store in exo_setglobalvariable, exo_getglobalvariable, can we store images etc?
can we use it as simple noSQL? key=>serialized value storage?

There is no known limitation. exo_setglobalvariable, exo_getglobalvariable stores data in XML format, and they should encode special characters. If you want to store images, you could save them in the special storage folder (HEPubStorageLocation global variable) : see the General Demo or the User Guide. If you want to protect them, you could crypt stored files.