Been using this for a month or so with only max np on 4 accounts including my main so far seems very safe. I think dmg mutilper on small setting might be safe too(2~4x) but never tried. I tried win in 1 turn before but felt super scared so never bothered again.
Only bug seems to be that you can lose all ur np if u crit on the third card of an art chain or if you crit on the art card when np is full