What isn't there to say about the Natascha? Let's start off with its statistics.

On Hit: 100% chance to slow target
-20% damage resistance when below 50% health and spun up
-25% damage penalty
20% slower spin up time

On paper it seems like it would be terrible, and in most people's eyes, they really believe it, too. The Natascha's true strength, however, relies in your ability to be a real team player. You don't NEED to secure your kills with the Natascha, it's entire purpose of slowing down makes it fall into more of a supportive role, allowing you to spread your shots across a group of enemies in order for another class like a soldier to use their hindered movement to their advantage and deal a devastating blow to the sitting ducks. It transforms the minigun from something that has no viability at longer ranges, and uses it to become more of a fearmongering tool, a warning that all who get caught within your sight shall be slowed, and all who get too close will be mowed down. The 25% damage penalty really isn't all that bad if you're willing to commit just a little bit more than with, say, the tomislav or stock. Also, because of the way that the slowdown works, it ramps up the closer you are, allowing more of your bullets to hit the opponent while they're slowed, giving it an edge over it's own stat penalty, and making it more useful as a weapon to take down single targets. You don't need 20 scouts on the enemy team to justify running the Natascha, this works on any class. And it's slower spin up time is somewhat ignorable, so long as you're willing to either pre-rev corners more often or stay spun up for longer periods of time.

But let's dive into its real strength. The line of text that EVERYBODY ignores. "-20% damage resistance when below 50% health and spun up". If I had a dime for every time a demo ran straight into me while I was revved up thinking he'd get his kill in 3 shots, I'd have enough money to buy myself a gold pan. It's such a simple, yet such an effect line in the weapon description that conveys all the info you need to know. So long as your opponent isn't using a one-hit wonder like a sniper rifle or a backstabbing knife, the fact that you'll take damage over time allows you to increase your survivability, and extend your killing sprees by a rather significant margin. 20% damage doesn't seem like that much, but when it cuts down on that afterburn, or lessens that would-be final pill/rocket, it gives you just enough time to kill while the iron is hot, and overpower your would-be killer. The universal damage resistance is single-handedly why the Natascha is just barely viable enough. Am I saying the Natascha is Gaben's gift to TF2? No, absolutely not. But I am saying, that maybe it's worth it to put the weapon back on your radar and give it a second chance. It's really not that bad.

The only drawback with it is that direct 1v1s with stock heavies are always a loss. But the Natascha can still outdamage the tomislav, so that's a plus.