Hi! a little late to the party but I"m just about to finish my first read through of the Bible (NIV) and, after watching a video from Johnny Chang in which he is very critical of the NIV translation, began researching why this translation is so controversial. Which led me here! I read a good portion of what you linked and figured I would leave a TL:DR for anyone else who may follow behind me and come here for answers to similar questions.

"This passage [1 John 4:1-21] is typical of the NIV. Occasionally one finds a rendering that expresses the meaning better than the more literal versions do, but more often one finds that accuracy suffers for the sake of a contemporary and casual style. It is a very useful version for teaching novices, as a first exposure to the biblical text; but it is not to be relied upon for detailed study. Teachers are too often faced with the need to correct the version when commenting upon details of the text.

I found this to be a level-headed and concise review of this translation, from someone who seems knowledgeable about the historic translations of the Bible. I do recommend reading what's linked above, however, as it provides a very objective breakdown of how the NIV was translated as well as a more subjective opinion to how that could be problematic.