Just yesterday I installed the same rule for my personal development.
Same story as everyone else, in a nutshell - I saw some nice-looking package (swift-foundation-extensions, in this instance), added it to my project, and then saw my builds suddenly taking way longer. Then noticed the dreaded "SwiftSyntax" named in the build logs.
What was a clean build time of about fifteen seconds became several minutes. 
("how often do you do clean builds, though, really?" - ugh, a lot. I'm using Xcode and it's got some bugs regarding detecting changed sources, not to mention its infamous bugginess regarding stale compiler errors)