No, the Fluid version wont load or sync correctly while offline, so that’s a drawback. But personally, I need multiple tabs and windows far more frequently than I need offline support, so it’s still a net win for me.
It’s similar to using a different browser, dedicated to a single website, yes.
I prefer to use native windows implemented by the OS than split screen panels implemented in the application, because I have global hot keys configured to let me do all kinds of nifty window management across applications. Much faster and more convenient to have consistent behaviour across apps than for apps to implement their own window emulation that forces me to learn different hotkeys for one specific app.