Get Element Unique Id in Exported IFC
Exporting the Revit UID may seem like a good idea because they tell me it is unique and never changes. However I once stored Revit UIDs externally for a model and then only about 50% matched up a week later in that same model. Had I changed 50% of the model in less than a week, no.
You may also notice that for Revit UIDs the majority of the UID is the same for many items (with only the very last part being unique). I'm thinking UIDs are partly related to initial workset assignment within a model and I'm thinking the reason only 50% of my externally stored UID's were found is because I detached and discarded the worksets!
There is a feature of the IFC exporter noted below. If you use this it will add a UID instance parameter to the element being exported in Revit and populate it with the UID used for that element in the IFC. If an IFC is subsequently exported it will use this UID from the parameter in Revit for the UID in the IFC file i.e. it can be used to track the same element between IFC issues. This to me seems like a better option.
You can access these exporter options via the API but it is not straightforward since you need to get objects from the custom exporter dll (depending on Revit version this is located in different places). You then use these exporter objects to update the DB.IFCExportOptions. The exporter objects use IFCExportOptions.AddOption to add the options that the custom exporter will then refer to when you call ultimately call Document.Export(String,String,IFCOptions).
ElementIDs uniqueness is even worse since it is just a count of Elements being added to the database which then get consolidated during synchronisation.