I really think that the Mejai's Soulstealer is overpower and especialy compared with other snowball items. I want to talk about snowball items balance. Here are my observations, Soulstealer is way overpower compared to the Leviathan first then the Sword of the Occult.

First of all, its easier to kill heroes with AP than hp and get stacks.

Second, the 'Gold value' of each stack are really unbalanced.
2 stacks on Soulstealer = 20 AP = 435 gold.
2 stacks on Leviathan = +/- 65/70 hp. 180 hp = 475 gold. It need 5-6 stacks to = the gold value
The Sword of the Occult is a bit different since the value of the + crit dams looks high (Infinity Edge).
2 stacks on SotO = + 4 dams and + 4% crit dams. + 10 dams = 415 gold

My point is that Mejai's Soulstealer can caps at 220 AP for 1235 gold, Tier 3 AP items have 70-100 AP.
The Leviathan hp bonus caps under 1k hp and every stack gives like half of a lvl-up hp, nothing to compare with the Soulstealer.

Before people counter-talk, I have to say that in many cases and without an AP build hero I buy the Soulstealer because even with 6 stacks, +80 AP it's alot and with more stacks, +140 AP for one item slot it's insaine. I'm not posting this because I'm tire to be farmed by some +300 AP Eve or things like that. I only want to talk about it ^^.

20 stacks on soulstealer = 15% cd. Not that great
20 stacks on Leviathan = 15% damage reduction. Great

Actually, considering the uniqueness of the stat, Occult Sword would have to be the best snowball. Percentage gains to damage are much better than lowish flat increases.

SS is also going to be found mostly on casters, typically the squishiest champs, so stacking charges is tougher for casters.
Occult is better for dps'ers (tho i think the stats it offers should be damage/attack speed or damage/crit chance, but w/e), who typically are a bit more durable, meaning easier to retain charges
And Levi is made for survivability, making it theoretically the easiest to maintain charges on.

In this respect i think they're actually relatively balanced from the cost perspective.

Mejai is balanced, Levithan's bonuses are mediocre except for the 15% reduction at 20 stacks which is awesome, and sword of the occult is very weak and needs buffing because it is completely overshadowed by regular DPS items such as Infinity Edge and phantom Dancer.