What are the dangers of running Flash on Mac?
While it's a bit dated, the below linked document may help address your question. This letter titled Thoughts on Flash was written by Steve Jobs in April 2010. While it was written to address why iOS doesn't support Adobe Flash, few points are also relevant for reliability, security and performance on macOS. I have quoted them below:


  Symantec recently highlighted Flash for having one of the worst security records in 2009. We also know first hand that Flash is the number one reason Macs crash. We have been working with Adobe to fix these problems, but they have persisted for several years now.


As stated by Apple, security and reliability concerns, and availability of open and standards compliant alternatives have propelled them to discourage use of Flash. Flash still continues to be available for macOS for those who need it.