Well, its not hard unless the opponent is actually good =P
Let's continue with the Metaknight example. Assuming you push one off the stage, what do you do? They will airdodge your projectiles, so you can't just randomly throw them out. If you try to spike them, they will reverse-shuttle-loop and then *you* will be dead and they'll be back on the stage. And if you *can* think of a way to reliably gimp MK off the stage, he can just glide under the bottom and recover on the other side.
If you can't kill Metaknight off the stage, that means you need to kill him while he's still *on* the stage. Samus cannot, from what I've seen, get a reliable KO off the top until about 180+%, and that's being generous. Pretty much, you are fishing for fsmash/charge shot KOs, and you can't really do that from the stage until 150+%.
Again, she's not *bad*, but she will not be winning any awards for awesomeness, either. I actually think she's underrated, but that just means I think she belongs near the bottom of mid-tier, not top.