The point is that when a lock is 'very hard' you cant even attempt to unlock it until your lock-pick skill is 100. Unlike Oblivion where the different difficulties actually just meant how difficult it is to pick the lock. It didn't dictate whether or not you could even attempt to pick it, you always could.
While I do like this approach better, there was still an inherent problem with it. That being, in Oblivion, the lockpicking minigame was way too easy. Any experienced player knows there's a certain pattern to go by in order to pick a lock, and once you figure out that pattern, the security skill becomes essentially useless. I could pick a very hard lock on my first attempt with 5 security just because I know the pattern. Kind of ruins the purpose of leveling security.
Fallout 3's minigame is better in this aspect, as there is no visual pattern along with an audio one, but it's still relatively easy at times. If limitations were removed, I could still probably pick a very hard lock at 5 lockpick on my first try, only difference is that it would probably take longer.
I do think limitations for hacking make a lot more sense though. Explanation being your science skill isn't high enough to know how to operate the OS in the current terminal in order to hack it. Liken it to a Windows user trying to learn how to use DOS or Linux.