Figured I post a thread instead since it's not really a bug, it's more of a design flaw. Starting off with some of the first thing I noticed and maybe will add more later to this thread.
Cloud storage
Generally seems to work well, but there is a lot to wish for here.
No subfolders - Saving to cloud storage doesn't let you choose any folder or subfolder. It's not very convenient to have all your project/test and scrap files scattered in one folder. I usually keep my projects in their own folders, with subfolders for everything from inspiration pictures, PSD files, design documents, C2 project, project backup files files etc etc. This has to be fixed somehow.
Saving files - Currently in C3 if you only click the save button you get the save dialogue for cloud storage as default, if the file was loaded locally, which is kind of annoying. Besides from that, if you loaded from a cloud storage and just click the save button i would image C3 to just save the file quietly in the background without opening the save to cloud storage dialogue over and over. Not very pleasant experience if you're like me, a little paranoid and save quite often. You can't work while it connects to the cloud service and saves. This would be good if it was a bit smoother in the background.
Save to local browser
I don't like the "Save to local browser" option. I have no idea where the file goes, and same as above there's no folder options. Does these files get lost if you somehow get your cache cleared? I don't know where the file ends up, and it doesn't feel safe to me to save this way. This option could be removed completely in my opinion, I wouldn't dare to use it. The only good thing about this is that clicking save button doesn't popup, the save to cloud storage dialogue, but could still be done in the background instead of showing progress bars.
Download a copy
This also needs a "Download as..." functionality. Currently it saves to your default download folder. Naturally my regular download folder is packed with garbage files, and I usually clear it often. I don't want my C3 project files to end up in that mess, because as soon as I save them I have to move them out of there to another folder for safekeeping.
Opening local c3p files
I guess one drawback of being browser based is the file handling. The c3p extension isn't recognized by the system and double clicking it in a file explorer, naturally the system doesn't know what the hell this file is supposed to be opened with. I don't know if there's any way to ever address this issue. How do you tell the system to open chrome, browse to C3, then load the file? How to associate a file extension with a web app function? Although it seems to work to drag and drop the file straight into C3, but personally i never do it that way. I usually just double click the project file. And then again, if you open a local file and try to save, you get the option to save to cloud storage, it doesn't just overwrite your local file, but i guess that's pretty impossible at this point.
Working locally in general
This doesn't seem to be a very good option at the moment. I don't see saving files locally is any good at all except for maybe downloading a copy of your project as backup. Even the "Save to local browser option" doesn't seem very good or safe.
Autosave
I noticed the "periodically backup projects" option in settings. I have no idea where the backup goes and how often the backup saves. There need to be more info on that, and further settings.
Verdict
Working on the cloud seems to be the only viable option but it has to be done WAY better, with better integration and more seamless saving and loading of files, with folder structure etc. I'm used to keeping my C2 project files on dropbox, but in a nice organized manner per project. Not in a folder with ALL of my project/backup/scratch/test files all mixed together. Preferably cloud storage connections should be done when you open the C3, and stay open during the entire session.