Do npj and Nature-branded journals have the same reputation such that I can say I published "in a Nature journal" after publishing in npj? In 2014, Nature collaborated with parters and launched npj, and they state "Nature Partner Journals adopt a modified set of Nature editorial standards". In fact, relatively high impact factors are observed in these "npj" journals but these journals are branded with "npj (Topic Name)" rather than "Nature (Field)".

What's more, the journals like npj Computational Materials (IF~9) and Nature Communications (IF~12) are slightly comparable in terms of the impact factor.

So my question is, do "npj" journals have a comparable reputation to other Nature derivative journals like Nature Materials, Nature Physics etc.? Basically can I say I published in a Nature journal after publishing in npj?
The answer is a resounding no, they don't carry the same reputation. The npj journals are quite clearly on a lower rung of the prestige ladder. You can easily verify this by comparing the impact factors, which is >40 for Nature, and ~9 for npj Computational Materials, for example. It isn't only age either - the Nature Publishing Group clearly has an incentive to offer a range of journals for different topics and levels of impact, while preserving the reputation of their flagship journal.

As for whether you can say that you've published in Nature, see When someone says a paper is published “in Nature”, does that mean it's published in Nature? In short, you're much better off not misrepresenting where you've published.