I tried this the whole way through today.
[Note to self: “I said I would be less negative…”]
OK, so… one thing I found is that you can only upload an RDL (aka “Publish”) once per Download/Design. That’s annoying.
In classic I left the file open. I’d make a tweak or two, then upload and test, then tweak/upload/test, ad infinitum.
Now it’s “Download” > “Design” > tweak > “Publish” > test > Download AGAIN > Design AGAIN > etc.
Other randomness
For me, I thought Report Builder was not launching. Turns out the server popup was hiding behind other windows. Had 4 of them till I noticed. 
Is there a place to change the download folder (“local location”)? I guess it’s moot once you actually notice the popup… But I just have a habit of keeping the RDLs somewhere I can find them.
Also, I need to replicate the error thing as you started out with in this post, in order to also report it. [I am trying to be good.]