Sentences being too short and annoying to read is a stylistic grievance, not a grammar one. The examples were edited in after my original reply, but even then, I see no actual grammatical mistakes there.

Even with my native language, which is definitely a lot less represented in the training data, the worst I encounter are phrasing mistakes, incorrect use of idioms, and invented words. You have to use some really badly tortured local model to get an LLM to produce incorrect grammar.

I do sometimes see Opus make typos, which is entertaining, but again, not a grammar issue.