What follows is my opinion only, and I realize that it "goes against the grain".
If you need to install Snow Leopard because it has something in it that you like or need (to run older software, for example), just do it and don't worry about it.
Just a week ago I retired a 2010 MacBook Pro that came with Snow Leopard. I continued to use it as the "regular OS" on that machine for it's entire service life of almost 7 years.
Ran great, never a problem.
"Security" was never an issue for me.
Again, my opinion only.