Liquid cooling can be unsafe, however the risk is minimized if you do it properly, follow all directions, and maintain it as per manufacturer direction. AIO Coolers like Corsair H series or NZXT X series are good liquid coolers that have very low risk of springing a leak.
That being said, the majority of people will argue that air coolers are just as good, and they probably are. I went for a Corsair H50 in my rig because it was on sale for $40, and I wanted to give Liquid Cooling a try. I'm very satisfied with my purchase, as it's both efficient and quiet, but some people will suggest the CM 212 EVO over the Corsair H50.
In the end, It comes down to preference, I find.