It's complicated.

SMBs can find an audience that would otherwise be impossible to reach. The thing that is advertised is so diverse it's hard to generalize, however the main thing I've noticed is SMBs live and die by the quality of their ad creatives.

For big businesses which make up the other 50% of Meta ads revenue: They are very effective in circumstances where your main offering is operational and the long term value of a customer is very large. For example it would be a very good place to advertise health insurance and TV subscriptions (which is a significant amount of the big business spend). Here I am more certain. If you didn't invent your product, and it is a beneficiary of the zeitgeist promoted by social media itself (beauty-focused, soft core brain rot), and your thing is basically fungible, you will thrive.