Ghasts atm are tooooo big to be unfair. Many little ones swarming would be unfair. Or a dragon ghast O_O that cant be hit by anything But a golden sword.
Erm, that was kinda how they were originally according to Notch's twitter/blog... They were once smaller, and immune to all but fire damage. Apparently they were seen as being too unfair. The pigmen were likely added as consolation and testbed for village behavior.
ghasts aren't hard because a ghast by itself is just hard, they're hard because they can pop around the corner and start firing at you at any time - when youre in a fight with pigmen, if you're surrounded by fire or even lava and don't have many directions to go in, if you're on a ledge or anywhere else where you can't afford to have the ground exploding under you, etc etc... they're creepy because they're always floating around and can fire from extremely far away and there's nothing you can do about it usually, in an open space anyway.
but also i do think the nether is too bare bones. one thing about mobs in the regular world is that they get geometrically more complicated when more than one type is attacking you (spiders jumping out at you while you're in the middle of a firefight with some skeletons is particularly annoying.) the nether just doesn't have enough going on in it yet. ghasts could be a much scarier part of a bigger picture if there were more mobs in the nether (the one other mob is potentially neutral if you're safe). i don't actually think ghasts should be changed much - at the most maybe they could have more health for when they come into range. rather the nether needs to be built upon some more and the ghasts would just be one part of how terrifying/hard it can be.
honestly imo the pigmen are way more 'unfair' - if you so much as look at one wrong you've suddenly got 20 ****ing pigmen running after you.
Hunting for Brightstone I repeatedly took damage. At 2 hearts left and out of food, I found myself completely lost, having to try to figure out how to get down without digging my way all the way down, wasting my diamond pickaxe and leaving myself without shelter. I ran from cave to cave, desperately trying to find my previous path, running inbetween areas all ablaze due to previous Ghast bombardments while of the over a dozen Ghasts at least half spotted me and bombarded me from way above, while I could not try to fire at them because I'd risk running into fires... Finally I recognized a lava stream and did a hard turn right, running inbetween fires, taking falling damage and all that, to reach my tunnel down to my portal at only half a heart left. Fair my tail, I got too many open areas to properly put out their fires or fight them, every time I kill one two more pop up.