Worst is definitely hyperbole but I think you could make an argument for most disappointing.
Metroid Prime is considered one of the greatest games of all time with the 2nd & 3rd entries being very highly regarded as well (metacritic scores of 97, 92 & 90) so the expectations for a sequel were very high.
Adding to that, there was an entire decade between the release of MP3 and the announcement of MP4, this huge gap caused a lot of hype for the return of a fan favorite.
A couple years later it’s announced that development isn’t going as planned and is restarting under Retro Studios, the creator of the first 3 titles.
Finally the game comes out 18 years after the last entry and it’s a pretty good but not great game.
To summarize, it’s not Nintendo’s worst game, not even close but the combination of being a follow up to one of the greatest game trilogies of all time, having an 18 year gap between releases and 8 years of development meant that anything less than amazing was going to be a huge disappointment.
When the herd loses its way, the shepard must kill the bull that leads them astray.
