If you use Mods, if the mix is not the exact same between builds, you will hose your saves going between them. Otherwise, it could cause some delays as saves are made and 3MB to 5MB of data is sent up. I also don't know how the mechanism works when it is offered Does the save get created and store online or is it created locally and sent up to Steam later. Also, there is no online component to this game, so I don't think cloud storage would serve as much a purpose. My Fallout 3 Saves folder is approaching 1GB, and I just cleaned it out a couple weeks ago. Comparing that to a shooter where saves are a few dozen KB or slightly larger. If they do, there will be a limit as most saves for this game start at 1MB and quickly grow to 5 MB by mid game, depending on what you do.
I have doubts that Steam will support Cloud Saves.