"...LLMs are good if you tell them the specifics (about the tone / context). Is it what you meant?"
They're inconsistent. I believe they were better 3 years ago. Today, they seem to be combining multiple sources and sometimes picking the wrong one. They are better at responding but translation took a slight decline.
You do need to give specific instructions on how to respond. Some of the cheaper ones seem to ignore instructions... GPT, Claude, Gemini, all three make this mistake.
I also mean there's a lot of room for improved quality and nobody is tackling that either.
"And any idea why people still use GT/DeepL? Is it the speed and cost? Risk of hallucinations? Or some features LLMs miss?"
I guess for the reasons people use Wikipedia. GT is verified by humans, including some experts. DeepL appears to be an AI and it does some languages well. It's a safe choice and popular brand.