I think you're forgetting about UB's innate Attack Speed bonuses.  All UB has to do is stack life steal to the max and then it's good luck killing him if he closes the gap with you.  His attack speed is insane.  Hopefully the developers will fix Life Steal's mechanics in an update.  Solace you tell me if you manage to beat a UB stacking life steal gear.  Until then, good day sir, and I hope you don't run into one at close range.

You lost the game long before that happened if he did.

I don't think life steal should work if you're not dealing damage... Kind of makes it regen rather than 'steal' doesn't it?

It's an aesthetic thing though, nothing to do with balance

... life steal is a %, not a proc. My point is, if he's gaining more life from life steal than you're damaging him, he's also dealing three times or more to you, and you're gonna be dead in three seconds anyway. In that condition, yes, you're dead. It's got nothing to do with life steal.

Just a tip, but if by any chance he had girdle of giants, he could have been feeding off your minions and nearby creeps happily, while still pounding away at you (100% cleave chance? i do find it extremely powerful with the UB and life stealing)

From what you are saying, i assume this was late game, so having such an item is pretty much a possibility.

Me myself, i havent noticed draining health from enemies that are shielded, then again.. those shield do not last too long, and im usually more concerned with the damage im doing, than the health im stealing

I find lifesteal very underwhelming until lategame, it only seems to work on melee hits, I don't see it coming from abilities.  Would be nice if you use an aoe attack on a kreepwave and get a percentage of that in HP.  Does it work on procs?  Might be nice with Ashk if it does.

As it has been said life steal items are so expensive that you might as well have lost by time mageslayer or ashkandor come around.  (And it sounds like the UB had both)  These two items up weapon damage so much you would die without the lifesteal anyway.  Besides, it does take up 2 armor slots, meaning he had to toss away something like health to get them.

Foul grasp doesn't do lifesteal from items, it's part of the skill itself.  It goes to 664 damage max rank, although I only get one rank anyways for the interupt/stun.  And once oak shields nothing can damage him, however foul grasp will still stun him (Take that you shield/heart of life oak wannabies!  ) and bite along with other abilities will still slow him, but he won't take damage.

Most likely the battle went downhill fast because the Unclean Beast had high level Ooze. He could have been reducing your whole army's attack speed by 40%. You just think that Lifesteal was the problem but really it was the fact that you're only dealing half as much damage as expected.

I'm sure that this is the problem since you got back up with Last Stand. The only way you wouldn't cleave him is if your attack speed was dramatically reduced by Ooze. He probably killed all your minions with Ooze as well by the time you went Last Stand. This reduces Oak's personal damage from Soul Power as well as eliminating all incidental damage from the minions.

All things being equal, Oak should not be able to take out Unclean Beast in a straight melee fight. Oak is a general going up against the absolute best melee assassin in the game. Or do you want the all-around characters with lots of options to beat the specialists at their own game? Kind of hard to see how that would be balanced.