I'm going to toss in a definition or two. The word "sharing" - as in "file sharing" - has multiple meanings depending on the intimacy of sharing.
To put a file where it can be downloaded to/by others IS definitely file sharing, but of a very limited nature. The sharing in many such sites is to use the web page's ability to pick up a file via FTP or SFTP, a File Transfer Protocol. This is a WHOLE FILE transfer protocol. I.e. you pick up whole files at a time. Drop boxes use this ability of browsers (to use FTP or FTPS mode instead of HTTP or HTTPS mode) to claim that they are a good way to share files. And they are not technically wrong.
But there is another level of file sharing and that is the intrinsic Windows File Sharing. This uses the SMB (Server Message Block) or secured variants thereof. When you use SMB, you are able to share components / contents of a file without sharing the entire file in a single operation. I.e. piecemeal rather than wholesale.
The reason that FTP-based transfers work OK even for large files is that FTP includes the ability to request a re-transfer of a block in mid-stream without voiding the entire process. The details of the protocol include the ability to take several blocks at once and reassemble them in the correct order on the receiving end of the transfer. However, if you lose a block with SMB, it doesn't do so well and the transmission sequence becomes garbled. SMB does not recover as well from lost blocks mid-stream.
Access, when it is dealing with a "native" back-end file, depends heavily on SMB. If the back-end is some sort of smart "active" app like SQL Server, My SQL, ORACLE, or one of the other ODBC-capable products, there is yet ANOTHER protocol that is common to all ODBC products. Both SMB and ODBC require robust networks. Drop-box systems don't like - but CAN survive - flaky networks.
The issue of using SMB over the cloud to get to a back end file is that the cloud more often is less robust than a hard-wired Ethernet or Token Ring connection. And every time you encounter one of those connection breaks, you risk file corruption.