That's not just Emojis. Consider ANY language not using Latin at all, or rarely except for Brands (there are many such languages), then you need UTF-8.
Russian, Bulgarian, Ukrainian, Arabic, Persian,
Urdu,
Hindi, Bengali, Tamil, Chinese, Japanese, Korean, Amharic...
Much more than half the world population rarely use Latin or have strong difficulties with it (so much that even if brands in Latin are displayed, advertisers need to transcribe the brand names; even a lot of people don't have any Latin name registered for them, until an authority decides to give them such romanized names (they can't always choose themselves even if they can read Latin) to print on their passport if they travel abroad.
Of course programmers (in Lua or other languages) should know Latin, but for their daily work they'd like to use their own language for their own projects. It's quite common in China to have project names, variables, and APIs written in ideographs (and if they ever try to name them in English, the English produced is very poor and full of typos, and minomers; except for Chinese living in Hong Kong, just ask to those in Beijing and Northern China, or even those living in Taiwan that don't even all speak easily Mandarin, but other austronesian languages; go to Africa, even in those countries where English is an official or old colonial language, it is not always chosen; go to Egypt, they dominantly speak and write Arabic)
Arabic (or Hebrew) in programming languages however can be challenging due to the Bidi reordering that could mess a program (e.g. with Lua's curryfied syntax, or because it would reorder a simple division or substraction...). There are specific issues described in Unicode for Bidi scripts in programming languages. But beside this there should be no issue at all for Chinese, Russian, Hindi, Japanese, or Korean...
