In all honesty, I have been running 3.0.1 for over 10 years and have never had a security breach, hack, virus or other problem.
Maybe it's because the host company has good firewalls? I don't know, I am just a user with just enough tech savy to end up on boards like this asking what you all think are silly questions!
You do realise that by going that path that PHP 5.4.* is no longer supported so your host will be on their own and, come July 1st 2018, there will be no support for phpBB 3.1.* so you will be on your own.
"The good news is hell is just the product of a morbid human imagination.
The bad news is, whatever humans can imagine, they can usually create." - Harmony Cobel