Pro Versatile platform
You are not limited to the given selection of themes, you can upload third party themes, or even code your own if you know how to code.
Additionally, they let you export the site files and code. so it's portable. There are plugins/programs that can, for example help you transfer your Weebly site to WordPress, etc.
Overall, its a very newbie friendly website builder that is versatile and expandable. It provides all the necessary elements for building a quality website.