What would make them spiritually damaging?
I would say that it’s not so simple as “canonized saint=spiritually beneficial writing.” There is writing by saints that can be quite spiritually damaging if you’re not capable of receiving it well. And there is writing by non-saints that is very spiritually beneficial. Lewis’s The Four Loves was very helpful to my discernment. He’s not a saint. And some people read Liguori and struggle with scruples as a result of not knowing how to properly interpret him.
There’s nothing magic about a book by a saint that makes it good, nor a non-saint that makes it bad. That’s just not how books, minds, or souls work. If we want to know if something is going to be spiritually damaging or beneficial, we should look to see whether or not what it says is true, and whether it’s going to be helpful to us as individuals. As with everything, mileage may vary.