First I'll start by saying that not every autistic person is the same. It is so varied that I don't see it staying 1 thing forever, it will probably be split into multiple things in the future. So I do not speak for every autistic person, though I suspect there are more like me.

Emojis rely on a shared social context that I am often missing. Being an introvert reinforces that social gap. I read books far more than socializing, especially when I was younger. I have to put in a fair amount of effort to pull in emoji meaning out of context from the words around them. Any entirely emoji post might as well be a foreign language to me, think like spanish to a north american who isn't bilingual where you still know numbers and a few common words.

I have a static translation map in my head of emojis I have encountered often. A few I figured out on my own, a few I had to ask someone and make a mental note.

The face emojis are tough. I had to ask my wife what 🍆 meant.

Gifs tend to be richer in information and I don't have the same issues with them.

TLDR, I need more social context to figure out the meaning of social and emotional information. Emojis strip the meanings down to the bare minimum for normal people. That isn't enough for me in a lot of cases.