To simplify:
Grammarly is used to correct grammar.
AI is used to correct grammar.
Both serve the same function. FOR-ME
To simplify, Grammarly is okay for everyone on the forum, never heard any bad words about those who use it. AI bots are often used to cheat and abuse, many don't like any of their usage on the forum. Why to choose the tool which is not okay for many? It is suspicious.