Cons
-more expensive
This is a major, major con.
Average per-game income is only ~11k.
Which tends to work itself out to three major 3k items, 1k in boots, and more than likely 1k in the bank after the winning push (or spent on consumables/boot upgrades).
Slot effeciency offers no real advantage in LoL - more expensive items are for all intents and purposes simply more expensive items than less expensive items.
And blowing 900 gold on AD and life steal for a swanky active can put you 900 gold short of more important actives/passives to a given champion's play style - such as those of Zhonya's, Deathcap, RoA, or even something like BV or GA.
In an average game - you're actually sacrificing access to completing your 3rd major item - often removing that item's unique qualities from having any impact at all on a game.
And on this note - items being OP or UP is almost entirely based on the stats they give For Their Cost, not just a raw stat value.
A 30,k item is pretty UP and useless regardless of what it offers, it could offer 4,000 AD and still be useless due to offering 0 AD for 99.99% of every game it was persued in.
A second major consideration of items, outside of cost, is their build path.
LoL isn't a MMO raid of max-level heros.
Enough momentum can be won or lost to decide a game in sub 3k gold-earned phases of the game, where only component items play a major part in how that momentum is destributed.
And a Revolver + Cutlass combo is a pretty weak build-path for most champions ability reliant enough to consider investing in spell-vamp.
You're better off getting mana sustain in most cases and just having all your abilities ready to cast to minimize the danger/damage you expose yourself to in the first place: because any champion ability-reliant enough to consider spell vamp is pretty much completly toast if they run OoM
(You may notice every champion that regularly grabs spell vamp is either manaless, or has access to using spell vamp without expending mana).
Finally - LoL's itemization is all about optimization.
Marksman can build full tank items and use all the stats full tank items offer.
All but 6 champions benefit from AP, and all but ~18 benefit from mana.
Every champion can utalize a glass-cannon AD build and all stats offered in such a build.
What pushes champions to build as a tank, marksman, mage - be a tank, marksman, mage - is very much about subtleties in how efficiently gold spent on stat A is used over gold spent on stat B.
Consider 
has a +55 AD steroid and builds AP/support, where 
has a +55 AD steroid and builds AD glass-cannon, and 
has a potential +300 AD steroid and builds full-tank.
is a monstrous 36% more expensive than 
, which is a significant difference in gold costs and efficiency.
And spending 36% more gold for an item that's a little better on paper due to a cool active? Can easily be not worth the gold cost.
in several games items are balanced solely around their gold cost.
Don't rule out how powerful gold costs can be for item balance, the cost difference between and is pretty massive.
