The product development time was very short compared with other products in this field, especially considering that it's open source! I'm really happy about how easy everything worked out from beginning until end (and even after!).
There are no downsides at all - we needed an API backend specifically because our frontend application had some limitations when working directly against MongoDB or MySQL databases via their drivers/ORMs but now they're solved by using tinyBird as well. It makes sense that there isn't much more than what you could try yourself without paying extra money. We simply implemented one endpoint per database table which allows us full flexibility regarding querying both structured and unstructured content within seconds rather then minutes like before where only simple queries were possible due to slow response times.