While all the other damage talents seem to have a draw back of sorts, dark harvest does not. It can be used in lane or in the jungle. The stacks seem to be faster in lane, but if you time your ganks right, you can get 5-10 stacks each time, without getting a kill.
This isn't what makes it strong however, it is the 0 cooldown usage. It requires you to have recently collected a soul (unless you have 100+ stacks then it is a 300 second timer). This means that when you are equal or behind, you won't see too many issues short term. This probably won't affect LCS either, but it will dominate solo queue. The raw damage added is too high when you consider you can get easily 3+ procs a fight and if you gank a 2v2 lane with rengar with just a warrior enchant, you will likely one shot the first person then do 200+ bonus dmg the second time.
With just duskblade, warrior, youmus, and cleaver, I could jump out of stealth and auto attack ONCE and kill a full hp mage, adc, or support. This is worse with Shaco as his auto will likely be a crit meaning it scales the damage higher.
Pretty much any champion with a skill reading "applies on hit affects" can one shot you if they get any advantage while using harvest. I wish I had videos of this, but my plays.tv isn't recording my beta games. The reaction time is in the miliseconds for survivng a rengar ult auto, but there is 0 reaction time against a shaco auto or ezreal q doing 2k+ damage.