I'm part of the team that makes Zoho Writer's grammar checker[1] and we did go with a modern neural-net based NLP technique to detect grammatical errors. It had a huge win in terms of the number of errors it caught, but we could never tell the user what exactly is wrong with that sentence, because.. well, deep learning.
So we've taken a hybrid approach where a modern AI flags errors and then run it through a rule engine to try and figure out what exactly is wrong. This way we get the best of both worlds. That lack of explanation is also why Grammarly is sticking with GOFAI systems perhaps. But the future of this problem area is certainly AI augmented with rule engines.