As a front-end developer I just can't warrant the time setting up a backend, setting up a database, defining routes, handling authentication and the other nonsense that seemingly takes a solid day to get working.
Firebase to me is the best choice for simple projects, I don't use anything else anymore.
The only downside I really see is once you leave the free tier you're at $25 / month minimum whereas I can run a small backend (or multiple small backends) on a $5 / month box. The difference really adds up if we're talking multiple small projects for multiple years.
Technical downside: backend processing and search & filtering are more involved.
Personal downside: Though I code in JavaScript, I'm no fan of it. Is there a good library in Go for it?