I haven't attempted latin translations, but anything from my native language to english and back has been 100% perfect, miles better than anything google translate can do
Latin is tricky though. Google translate is notoriously bad at latin grammar, much worse than most living languages.
Not exactly sure why, maybe small corpus, maybe because it's a pro-drop language without fixed word order and an very complex set of conjugation rules.
That's probably most of the problem. The entire corpus of ancient Latin is on the order of 9-10M words. If you printed it at the same density as a typical English novel, it wouldn't even fill a bookcase.
Why would the corpus need to be limited to antique latin? The language was in use for much longer than antiquity. You've got everything from Thomas Aquinas to Isaac Newton to pad with.
