What does BuzzSumo do well? This is the best software tool I've used to discover popular content and to come up with new content ideas. It is also great that it shows how many social shares a content piece gets. This is very useful information when writing content that people will see.
What does BuzzSumo not do well? I don't find the backlink tool as good as others. For instance, I find it doesn't pick up as many backlinks as Ahrefs. Also, it is a pity you have to pay more for the Facebook features as I find Buzzsumo a bit on the pricey side already. It is also interesting that their influencers section focuses more on Twitter and Facebook when Instagram really is the platform for influencer marketing, at least for B2C.
What do you wish you had known about BuzzSumo before using? I wish I had known that some of the features aren't as good, in particular, the monitoring feature. Buzzsumo sells itself as an all-round package but it's not really. You need other tools for your business as well.
Have you used a competitor to BuzzSumo? If so, how does it compare? No. However, I do use Ahrefs for monitoring and I find it more accurate than Buzzsumo.