Do you mean "safe"? ("security" is about criminals hacking your phone, "safety" is about not damaging things or self).
I'd say it's like with PC overclocking - if you don't do anything crazy (like increasing voltages too much) then you're safe.
From what I can see, we can overclock our phones with voltages *lower* than stock, so that's as safe as it's going to be
There's also a related safety issue: if your phone crashes, its filesystem "might" become broken and the phone "might" not boot. I haven't seen that happening, but having a nandroid backup is just common sense.