What does Google Sites do well? Google site is extremely easy to use and you can setup a website in minutes. That will be linked to your Gmail account (either personal or a G Suite corporate one). If you're using that associated to a G Suite account you can also give rights to other users to edit certain sessions of the main website. While editing your site, there is a sidebar where you have multiple options to help you while building your pages.
What does Google Sites not do well? Since Google Sites it uses it's own web interface to allow the user to build its webpages, that limits a bit the way we can add custom elements to our pages. I believe that this is a good option for static websites where you basically have text, images, and hyperlinks. However, if you're planning to create more complex dynamic websites, I'd look for a host that could offer you more flexibility in terms of resources.
What do you wish you had known about Google Sites before using? Every time I use something from Google I expect quality at a low cost (or even free). This time it was no different.
Have you used a competitor to Google Sites? If so, how does it compare? I haven't used any Google Sites competitor in years because Google's product is so good and it's also free that it's really one of the best options for simple websites.