One thing, the steamie doesn't grow defence. Assuming you don't quint them, it is going to be dead at one point. A golem can survive an infection, even a double infection. There is also a limit on the amount of potions you can hang on the steamie. You can keep boosting a golem if you like. A golem with 40 attack is going to be sitting on 34 defence if im right. A steamie can get 50 attack, and its still sitting on 15 defence. Defence counts for something. The difference in attack is not that big.
5 grows: Golem = 17 Steamie = 20
10 grows: Golem = 27 attack. Steamie = 40.
15 grows: Golem = 37 attack. Steamie = 60
That is assuming you have all that fire. I usually don't have it, then the stats drop again. These stats only start to matter if you've already invested some 10 turns in the steamie. It still needs to catch up for the damage the golem already did of course.
In the long run, yes, it is a great unit. For speed, it is awful. After all..most players will be done for after 10 attacks. Given these pros and cons, it evens out. The units have different uses is all.