So I just took the 70-778 exam online, and I passed! I obviously can’t say what was on the exam but I will say a few thoughts I had about the preparation for and process of doing the exam itself.
In terms of preparation 80-90% of what prepared me was my experience with the Power BI. This isn’t a quick way to learn but it is the best and there is no substitute for it. I did buy the accompanying book by Daniil Maslyuk, which did help for some of the areas I wasn’t very experienced with. For example loading excel workbooks into the service or the administrative side of Power BI. The final piece of preparation that I found useful is a quiz that was made by Rishi Sapra. It has similar questions to what the exam contains. It is a bit out of date now, but it is still worth doing at least once.