Little grain of salt means you go into it expecting error or at the least the possibility of error, I don't get why someone would recommend writings which border on erroneous theology. Would you recommend something like Valtorta like Bp. Williamson and just look over the error (to put it lightly)? Makes no sense.
I don't think Emmerick's theology is erroneous, just that I believe some of the narratives regarding the details of Christ's Life should be understood as a combination perhaps of fact and fiction, a novel attempting to depict Our Lord's life in vivid detail combined with some actual private revelation ... so a blend. Some of this embellishment could be Emmerich herself "interpreting" something she saw, or thought she saw, while some could be due to Brentano fancying himself a great poet and novelist. That's not to say there's theological error in her works.
Valtorta is a different story entirely.