Well, I think this is one of the big problems with The Academy as an institution. A lot of shady stuff has gone on with these awards, and given no serious rival groups, there is very little pressure involved for it to maintain higher standards, and consistincy. Of course then again in the end this is all a matter of the opinion of a group of people, and opinions are by their nature subjective. Is it fair that someone's overall career or later work within a year can hurt their chances of getting awards? Not really, but then again in the end opinions by their nature aren't usually fair, and can be swayed by politics, whims of the moment, and everything else.
That said I think people undervalue Eddie Murphy quite a bit. He's talented enough where I think he deserves some lifetime achievement awards, and to be fair while he's had a good number of bombs, it's noteworthy that his failures have in many cases been better than the successes of some other actors (or so I have read), it's simply that he has yet to pull down many blockbusters despite people thinking he's always on the verge of doing it. I remember reading somewhere that pretty much every movie he's been in, including "Pluto Nash" actually made substantially more money than they cost, albiet over a longer period of time. He's had box office bombs, but even his "Terrible" stuff winds up getting the rentals and DVD sales.
I am ranting about this side element, because I think that Eddie Murphy's abillity to play multiple characters in the same movie and have them come accross as differant people is pretty cool, and quite entertaining. Though admittedly "Norbit" did seem like he was trying to rip off "Big Momma's House".
When it comes to Natalie Portman, I don't think she's likely to have many problems, and even if she misses the shot here I think she's talented enough where she's liable to put out some more decent movies and be nominated again. Truthfully I think Eddie Murphy's problem is that he's first and foremost a Comedian with a few cool gimmicks (like playing multiple characters, and doing it well), he's only rarely distanced himself from that and done serious dramatic acting. Natalie Portman on the other hand is first and foremost an actress and has done all kinds of movies accross the genere spectrum, I think in her case being able to do something like "Black Swan" and a romantic comedy almost back to back like that is actually in her favor.
I also share the concern that Natalie Portman might be overexposing herself though. As ironic as it might sound, she is increasingly reminding me of Nicholas Cage. Nicholas Cage is an actor that managed to show off his abillity to do a wide variety of movies and film generes, he however wound up doing so many movies even before his money trouble (I guess that he liked to work) that I think he started to annoy people, especially seeing as he didn't seem especially picky about what scripts he took. He wound up taking the blame for some bad movies where his personal performance was actually fine. Of course he got older, and due to money problems he hasn't been able to be more selective, very much being in the "will do any movie that will give him a paycheck" catagory nowadays. His personal financial irresponsibility/mistakes having nothing to do with his abillity to act.