Wanted to chime in an also express confusion over the limitations of the upsert block (as well as the upsert directive) and how both behave in ludicrous mode.
An example of what is safe vs not safe would be appreciated 
For example (in ludicrous mode), if two upserts attempt to modify the same predicate do they conflict? If one upsert queries for a variable that changes before the mutation part of the request completes, will it abort?