I've run Ghostblade tons of times on Fiora, ever since season 2, and experienced a lot of different match-ups with it.
Generally, the pros and cons of
+
vs.
+
boil down to this:
Ghostblade is all about early game. The Brutalizer is stupidly amazing for it's cost, and Ghostblade gives every single ADC stat besides life steal. It gives you the power-spike of IE + PD in 1 major item rather than keeping you weak until a 2 major item combo.
Ghostblade is also amazing in 1v1 situations given it's active, making it very well-suited to assassination attempts and duels.
Kingblade covers the no life-steal weakness and also aids considerably in 1v1 potential via it having an active.
I should also note - Kingblade and Ghostblade are much stronger 1v1 items than they are team-fight items.
Mobility isn't nearly as good in team-fight situations as defense, range, or crowd-control for staying alive.
AS is also pretty useless when you've got to constantly re-position and poke/kite, disengage/re-engage.
In both cases it's a lot more extreme and obvious on melee champions, but AS and mobility are still a lot better in duels than large-scale fights if you're ranged as well. Caster-marksman like MF are pretty amaze-balls in team fights, where AA-heavy, mobile marksman like Vayne are not bad ideas to re-roll in ARAM.
Finally - it's important to point out that Ghostblade and Kingblade are both crapped on by armor.
Oddly enough, flat armor pen is actually really solid against squishies and not against tanks. 20 armor pen against an opponent with 20 armor means an effective 100% armor pen and true damage. Against 100 armor...it doesn't.
8% HP shred as physical damage also only means 4% HP shred when 50% DR (100 armor) comes into play. Not to mention hitting an opponent for half damage means half the damage for life-steal to work with, or effectivly half the life steal. Or 1/3 with 200 armor, 1/4th with 300.
Not something you'd expect at first glance with %HP shred and armor pen items, but that build is hard-countered by armor. So - very much about assassination. And early game.
is much more about the late-game, team-fighting hypercarry route. It's clearly better 3 major items in, giving more consistent damage that maintains strength over drawn-out fights, and some AoE with that Shiv for added team-fight prowess.
The most important weakness to keep in mind with this particular build is this:
The average game is only ~33 minutes long, and the average income per player is only ~11,000 gold. According to LoL King, marksman average is only ~12,000, and down to ~11,000 on a weak-early marksman like Vayne.
Which ultimately means early, mid, and late game are broken down into 1 major-item milestones. IE's 3800 price tag is more than 1/3 of 11,000, and toss in boots, potions, wards, and Doran's and you might not even hit a 3 major-item build in an average game.
Especially when you consider a decisive team-fight, win or loose, generally means kill/assist bounty and base structure bounty you don't even get to spend - or it could mean having enough gold to spend while you're reviving and watching your Nexus explode.
Strong early is a very potant advantage to have when you consider teams like SSW as well. Usually they aquired their major advantage by the 20 minute mark, which is heavily reliant on strong-early power spikes to consistantly pull off.
TLDR:
is good for assassination/dueling and early game.
is good for team-fights, shredding tanks, and late game.