Valentina kit is what makes her ult strong. The fact that she has a heal dash and dmg in her kit allows her to take any ult and make it work. For example if she didnt have a dash, then steal mino ult will not be as strong.
Subjective, again it really depends on the player behind it.
There are lots of heroes that most people say can't be viable in this meta but it's not really true. I'm still able to get good results with certain heroes that are not in the meta. So it really depends on your mastery and understanding of the heroes in order to utilize them well.