Some of this advice is lacking. I've used world anvil and I've used other apps as I've looked for something that wasnt a pain to use.
So, first thing that has to be cleared is, are you the type of person that NEEDS to keep things organized?
Are you writing a long story?
Are you writing a series?
Does your story have many characters?
Do you have many minor plot threads?
Do you need to remember stuff that will come up 100,000 words or a 1,000,000 words later?
Do you actually like keeping things organized?
Answering yes to any of those indicates an organization tool like world anvil could be useful but then we have the next question.
Would you be against keeping a word document where you can just write everything down?
World Anvil is an organization tool that only really reaches its maximum potential in the hands of the person that knows how to keep track of things. It allows you to organize things in a more appealing way, but chances are that most people's World Anvil use would not be better than my microsoft word+ excel use.
So, to answer the original question. The scenarios where I wouldn't reccommend it would be if you're writing a one-off short story or if you're not the type that writes things down for later. If you're the type that doesn't want to learn a new tool, then I would also say not to pursue it.