I can't Export and Save, either in Project Siena and PowerApps.
It always prompt: Error in saving your files. Please try again.
I think if saving to Cloud (Excel) is slow and affected by internet, then Export (as zip file) to Dropbox / OneD / GD and then automatically imported whenever apps is opened or used by another party will serve the SAME Purpose.
(Only differenet: you can only access in Win 10 apps only).