What's wrong with traditional file shares?

SharePoint is best for Office docs. It's not great for anything else, especially binary files, or files that link to other files. And you'll probably run into file lock issues if they're edited by multiple people.

If there's a clear delineation you can use for separate sites, use it. For a ~40 person business we just had one site. For a ~4000 person business we had one for each business unit. It's just an additional layer. Quantity of files and folders could be a factor, depending on how much you will be using it.

Also, if you're in any kind of regulated industry, keep that in mind.

Main advantage to me is Granular permissions and segregation : each site can have different types of permissions associated and it's easier to delegate site management (you can let the user manage their site) while limiting blast radius in case of error.

I vote that both are terrible solutions in the wrong situations (as already indicated by the others).

The hard part is getting people to use them properly!

If 'the business' has some ownership of their data and its mostly officey-crap, SP is probably fine and it's easier for them to see that it's secured and take some ownership of it. Chances are though that lots of the business units has little dips into each others data on the fringes...

(I'm just burned by a multi-TB file share a whole hospital uses where no one owns any of it and it's a tangled Gordian knot. Goes back decades, sitting on a SAN that doesn't give much visibility, just de-dupe)

How much data?