I would not call this a review it's more about describing my feelings about this show. The following lines are mostly about my impressions from the seasons 1-4 but the rating also reflects the several episodes in the season 5.
It took me a long time before I started watching this series (yeah, this year, 2020). From the very first episode I got hooked. I loved the main characters, especially Ragnar. I don't think that he is a great actor but he just fits the role perfectly. I'm aware that this is very subjunctive. But there were others also. The cast was great. I loved the story. I'll be brief, during the four seasons there were very few moments or scenes where I told myself "why?", "silly", "cheap", "cheesy". So my rating for the first 4 would be like almost perfext, 9 stars.
Then Ragnar got killed I started watching the season 5 and it feels like somebody moved a rail-switch to a different position and the magic disappeared. Feels a bit like a soap now. The reason? IMHO it's about the shift in character focus plus the story. There must have been some big change in the production team. It made me wonder "is this the same show?", "what happened?". I don't think that it is only about the absence of Ragnar. The story arc is fragmented, there is a lot of scenes and moments when I ask myself "why?". Not sure if I'll finish the show, I'll see.