It is true that it is easily possible to get a distorted view of things based on just a few negative or positive posts. However we were on the first Amazon cruise back in December 2017 and it was far from perfect. We are big fans of Viking (and are booked on our future sixth Viking river/Oceans cruise) but they dropped the ball badly with this itinerary. I expect them to put a lot of effort into improving things (excursions need a lot of attention).
While on the cruise I made a point of asking others what they thought about this specific cruise (not about Viking, the ship or the crew - all of these were great). I only found one couple who had a positive experience. At least ten other couples were in the range of "so-so" to "never take Viking again".
I see you are first time cruisers. Be forewarned that this part of Brazil has suffered badly from their political and economic crisis. You can see evidence of past greatness but you also see a lot more decay. This is not something one generally expects on a Viking cruise.
As the year progresses I hope you see more positive reactions on Cruisecritic to future Amazon cruises. However, I think at this point you will easily see more negative than positive reactions to the past Amazon Viking cruises