What does Adobe Spark do well? Spark is pretty handy to get your brand assets set and then providing flexibility to create content for various channels. Whether you want a video or image it works well. It can take a while to configure your brand, but once it is done outputting content is a snap. You can create for facebook, instagram, twitter... pretty much all the main media channels.
What does Adobe Spark not do well? It is limiting. While you can brand your content with logos and colors, you still need video and images from elsewhere. Perhaps Adobe stock would work, but this is only a piece of your tool set, not an all encompassing tool to handle everything you need.
What do you wish you had known about Adobe Spark before using? I was pretty much aware of what it offered, but I was surprised by a few niceties and disappointed by a few others. For instance, it can be a bit tricky to figure out some of the pieces on your own, and relying on others to assist the creation of content is tricky.