Sync is capable of working with SMB file shares (which are also known as Samba shares, CIFS, or simply “windows network file shares”, for simplicity we’ll call all of them “SMB” in this article), though one should be careful when setting Sync up. In short, there are certain limitations and peculiarities about SMB shares which one should be aware of.
Sync, and a user who runs Sync need full permissions to the synced folder - an ability to read/write files, create folders, etc. If Sync lacks some permissions - it will usually cease to deliver files to the folder with insufficient permissions.
Sync subscribes to OS notifications to get immediate updates about file changes and start delivering changed files. However, if a synced folder is located on SMB share, it may not be possible to subscribe to file update notifications. Only SMB 3.0 and newer supports notifications (which means, that both your client and server should have SMB 3.0 to get working notifications). If Sync is unable to get notifications, it will only detect file changes during full folder rescan.
Microsoft introduced SMB 3.0 in Windows 8, though it may be installed to your OS with Microsoft Updates (or via any other updates if you are using non-MS OS)
SMB protocol allows to lock files for exclusive use by applications pretty much like on local FS. However, sometimes network connection can be lost or even the app locking file may crash (or fail to unlock it gracefully) which may bring some files to the state when they are locked and cannot be accessed. Further behavior depends on particular SMB service / daemon implementation.
Apps access to Samba shares outside of Samba
The vast majority of Samba simple setups cannot handle well the following setup: If any 3rd party application accesses files shared by Samba in some other way (for example, directly if app runs on the same OS where Samba daemon does), file may be damaged, or changes made by a 3rd party application may be rolled back.
This affects one of the most favorite Sync users' setups: syncing data between 2-3 NASes with Sync, while letting other users access it with SMB. This setup may lead to files getting lost or corrupted, as in this case when Sync is installed on a NAS and accesses files outside of SMB protocol.