TL;DR
Split large savegames into multiple files so they can be backed up to Steam Cloud.Detail
Steam Cloud appears to limit cloud-saveable files to 400MB*. If a game has a save file greater than 400MB, then when syncing, the game will be marked as "Steam Cloud Error" and the "CLOUD STATUS" will read as "Unable to sync". When I get the game under 400MB, it syncs again, but obviously, it won't stay that small; The Factory must grow.I think splitting saves into multiple files (i.e. a split zip file) would be the "easiest" fix. Having a user setting for the maximum size might be nice, but I don't think it's necessary if you know the Steam Cloud limit. Being able to toggle on/off would be good, since there are cases where a single file is ideal, like sending it to someone else, or manual backups.
Searching the forum I found this suggestion, which was implemented, so this is the natural next step, yes?
*400MB was the number where my game stopped syncing, but if I'm reading the documentation correctly, it should work best if you keep it under 256MB or 100MB chunks.