How does the Drake kill the Deathlord turn 5? He summoned it turn 4, no attack. Deathlord hits face. Drake can hit it for 4 on turn 5. Unless he wants to Soulfire it, he can't kill it till turn 6.
Handlock will always keep their Mountain Giants if they get them in their mulligan. So that effects the actual odds, as well. Also you can run the Deathlord into something on your turn. I did that like five games ago, killed a taunted Watcher that was at 4/2. It summoned a giant. I had a BGH in my hand. If you have a very reactive deck, odds are you have a very reactive hand, so that favors you. Did it on Druid, summoned the Warrior's Ysera, Poisoned Seeds. A Ysera who doesn't even give you one card? That hurts.
I've had it screw me over, of course, but in about 50 games it only summoned something that caused me to lose twice. Once has a Ragnaros, once was a Sunwalker that I simply couldn't answer.