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I've got a Stack-On 10-gun cabinet and it is fine for what you say; keeping guns out of sight. No, if somebody REALLY wants in I'm sure that it wouldn't be hard to open, but it will slow somebody down, and to me that is one objective to a safe/lockable cabinet.
I don't know about an AR-type rifle, but it held my .22 and shotguns with plenty of room. The shelf for the pistols is kind of small but it does ok. They are pretty well made and as far as prying the door open, I'm sure it could be done, but would take a little doing. As far as I remember, there is a lug at the top bottom and lock side of the door (I haven't opened it for a while since I bought a safe). Not sure of the gauge of the material, but it is pretty heavy. There are also holes in the back so you could screw into studs if you want a little extra security.
You didn't say whether or not you have children, but that is another reason that ANY kind of locking cabinet is better than nothing. Overall, I have been happy with it.