I am wondering how long D&D Beyond will allow scraping of content and use on other platforms. I know that for 4th edition there were a few parsers that could grab the WoTC text from the online subscription, but there were no authorized VTT then.
The one big advantage of FG is the very extensive 3rd party library of modules available. Roll20 has some content, but FG is very deep in 5e. Buy the module and it is ready to run right away.
A public facing API for DndBeyond is in the roadmap, they don't seem to have a problem with other tools leveraging their data.