I'm not an expert on SSAS, so I can't answer that part, but PowerBI lets you build a quick star schema and start analyzing data immediately. SSAS can do things like data security etc. It's not either/or. You can start with Power BI and upsize later to SSAS.
here's an article that might help a little.
Here's a video by Rob Collie/Avi Singh on PowerPivot/PowerBI to SSAS.
the big thing, I think, about SSAS is that it can handle much bigger models and you get security built in.