I have been a user of the Language Tool API as we're building (experimenting with?) a language learning app [1] and grammar correction is a key component. I really want to love it, and I'm so appreciative of the open source version that has allowed us to get going, but it's just so... limited. I haven't really dug into the code, but I gather that it is very rules based and that seems destined to fail because so much of language is context sensitive.
For example, it really can't determine in German if you're using the correct case or not for a particular verb. Or it can't really tell if you're using the right prepositions.
Ultimately, we ended up building a grammar engine just for language learners using transformers [2] + a version of errant [3] we ported to be multilingual. It's not perfect, but after a couple weeks of development was more helpful than LT.
2. e.g. this paper from Grammarly https://arxiv.org/abs/2005.12592, but there are a bazillion others