is there any difference between using Ubuntu Unity and GNOME Classic/Fallback? Just to be clear, I'm not talking about the Launcher or the Dash. Of course Ubuntu Classic/Fallback doesn't have the Launcher/Dash, but this is not the difference I'm talking about. I mean differences related to performance, features, functionalities, compatibilities, etc. These kinds of differences. I'm asking this because I've heard the Fallback Mode is kind of "incomplete" when it's compared to Gnome Shell or Ubuntu Unity, so I just wanted to know whether or not it's true, because if it's true, I don't think using Fallback Mode is worth it.
The standard Unity interface makes use of your graphics card's OpenGL capabilities to composite the different windows to form the desktop image.
The Gnome fallback mode doesn't make use of compositing, so rather than applications drawing to their own buffers and having the compositing manager draw those buffers to the screen, they instead draw to the screen directly. This is simpler and can the performance difference can be noticeable if you have an underpowered video card or the video card drivers are not up to scratch.
With that in mind, there is also a "Unity 2D" login option you can choose that doesn't act as a compositing manager. For the most part it should perform roughly as well as the Gnome fallback mode. So if you like the Unity interface but are having trouble with performance you might want to try that.
One thing is certain though, the Fallback Mode is more productive and useful than the standard, default session. You don't get the full repertoire you may expect, but there's progress, good, healthy progress. In one fell stroke, you gain some 50-60% of your expected desktop functionality, which restores a bit of sanity and hope. Theoretically, you could get your old desktop back with some careful work on extra features, backward compatibility and a dab of visual polish. Experienced Linux distribution developers could pull this off easily, rebranding the skeleton looks with their own unique touch. Once again, we go back to Linux Mint, which has shown the art of subtle visual transformation many times over in the past.