Safari 14.0 no longer uses flash. First question is the Mojave that most recent update of 10.14.6? Is there a reason you are not using Catalina 10.15.6 such as 32 bit apps? It was against the last such version that Safari 14.0 for Mojave and Catalina were tested against.
The new Safari is definitely wonky. Even in Catalina, it hiccups when opening, and quitting it takes longer than previously. I suspect it might be due to the new tracking features.