Also, I suspect the main cross-platform portability issues you're likely to run into with an app like that are differences in filesystem behaviour, and developing it as web app doesn't really help there.
Given that BinFinder only needs the ability to read/write files and scan directories, it should be relatively cross platform out of the box. No real platform specific features, such as soft links, tags, etc necessary. On MacOS the app will likely need to ask for permission to access the disk, that's the only cross platform file system issue I see at the moment.
That's a good point - still you'd be sacrificing product development speed for the low level fs access, so the author should really make sure they know how much of it they'll need.