How safe are extreme aerobatic flights?
The maneuvers listed are not really extreme, and it is certain that, catering for the general public, they will be accomplished at a relatively high altitude. The airplane is widely used in the aerobatic field and is generally regarded as reliable. Therefore I believe that there is no more danger than there would be if you were doing a regular flying lesson.
Edit: I have dug up some more numbers for the aircraft type. There has been one non-fatal accident listed involving an Extra 300 in the Aviation-Safety database for the year 2020. From 1992 until present there have been 82 accidents listed of which 42 were fatal, resulting in the deaths of 67 people, including spectators listed for this aircraft type at Aviation-Safety.
As far as aerobatic flying is concerned, you can refer to this article for an overview of aerobatic safety. Most accidents took place during airshows rather than during training.
Below is the NTSB's summary of aerobatic mishaps from 2008 - 2015.