Sturmtiger almost has as much durability as a heavy. It's really different from any TD in the game and more closely resembles a slightly-less-durable heavy with a very strong flamethrower (but it has better range and AoE effect than a flame thrower), especially after they removed the base penetration from it.

It's a great tank but trying to use it as a heavy tends to get it killed because it's slightly less durable than heavies and tends to run past almost EVERYTHING with that low range and rotation, often leaving it alone and unsupported if you send it out first like you would a heavy. I usually send it out after sending out some other tanks first so I have a better idea of the ongoing battle situation before I choose what lane she goes down.