I think it was a combination of factors, the different servers being used around the country and world, how it is routed, the Internet provider, Netflix's capacity and how they handle the bulk of user, how they route it and the device being used and, in some cases how old the technology is being used by Netflix, the Internet provider and the user.
I can say Charter Spectrum had similar issues when they sold users, like me, Cloud DVR service, but didn't build the capacity and was using old equipment/software for that purpose, resulting in buffering, pixelation and in some cases recordings disappearing. Thousands of people were complaining at the beginning of the year until Spectrum started upgrading their stuff. In this case, I think Netflix underestimated a popular live event and the result was what happened.
I found that viewing the streaming of the fight worked better if I was not trying to watch live but instead watching with a bit of a delay, say 20 minutes behind the live event.
All I can say for sure is I know it wasn't just Roku's having issues as I try different devices to see if it was Roku specific and it wasn't.
