I've never had problems with people escaping me when I play Leoric. The 4 second stun/slow is usually enough, plus just hit a target that's already slowed or stunned by a teammate. I usually just make Armlet then BKB and the game is usually in your favor or the enemies. His passive Lifesteal and Crit are ridiculous. Mid-game with an armlet his critting pretty damn hard. I don't think he really needs the extra int from force staff unless your team has no mana boot holders (would be very stupid). The passive stat gain alone lets you cast your stun a couple times in a fight late game, and mid-game all you really need is mana for a stun and your ult, assuming you have good picks for support heroes on your team.
I think the problem pub teams have when I lose is seemingly the lack of stuns we have. I've noticed a bunch of times that there'll be a lack of a good initiator and lack of strong stuns.