But if you'll excuse my ignorance, what are "emojis"? Unicode
characters? Sorry to say that they don't look so nice and if they
are unicode characters you never know how they will be rendered on
They're indeed unicode character, native ones.
So the rendering may depend on the target, for example, what I see in GMail isn't the same as what I see in Eclipse IDE. If we're willing to investigate usage of emojis, we probably need to build some SWT example showing the main interesting emojis and capture screenshot on all OS.
But actually, when adopting emojis, it's a benefit to not know nor care how they're rendered: you trust the OS, just like SWT trust the underlying rendering; and the OS brings the updates, the scalability, the accessibility support, the options and all the stuff to properly deal with those images, way more efficiently than what we've been able to achieve with embedded images. I think that usage of emojis really fit with the "Eclipse IDE integrates well in your OS" mindset.