For both the Mojave Security Update and the Safari 14 update, MacOS was still causing many problems for people running Mojave, as outlined by Mr. Macintosh.
macOS Mojave users who have installed the update are crashing while trying to use Migration Assistant, memory issues, slow boot times and finder stalling, many system.log enticing, and other problems. Complaints about the update began shortly after its release, but Apple did not pull the update until yesterday.
Those who had already downloaded Mojave Security Update or installed Safari 14 should soon get a fix in the form of updated software, which addresses bugs that were introduced. Re-installing MacOS Mojave, or re-installing tomacOS Catalina also appears to successfully resolve problems from time to time.