What does Google Sites do well? It's a pretty straightforward and the drag & drop is easy to learn, even if you're absolutely new to website creation. It's also convenient when you need a website where you can embed your documents from Google Docs, Spreadsheets, Forms, and Slides. The features are quite extensive and fit most of the landing page requirements.
What does Google Sites not do well? Since I've mostly used WordPress, I miss the ready layouts, where you can simply edit the text instead of creating the whole thing from scratch. When you have a design in mind (or created by a designer for you), GoogleSites can be a great tool for the design's implementation. However, when you want minimum work to have a website, I wouldn't use GS.
What do you wish you had known about Google Sites before using? There isn't anything, really. Google Sites are a convenient tool. Even though I usually use WordPress, Google Sites are just different.
Have you used a competitor to Google Sites? If so, how does it compare? I usually rely on WordPress (and for work purposes, I've also used Wix) and I like the ready-made templates that I can fill out. For my purposes, those templates were much more convenient than creating everything from scratch (despite Google Sites being quite simple in use.)