Of the mech mods that I've seen, had or read about the only one that seemed to be "drastically" different in some way is a mech mod that has some sort of different locking device, (can't remember the name of the mod off the top of my head) that when you turn the base, it moves some sort of internal plate to lock the fire switch from being turned on.
I think the people above me have covered it pretty well with what each piece does and how it can be different, but in it's absolute basic form, they are all pretty much the same. A tube with a firing button and a top cap. The firing button allows power when depressed to go from the battery to the top cap and ultimately to the atomizer.
It's really more about presentation to me. Some are works of art (although mass produced art but art none the less) and some are quite utilitarian looking. Something for almost everybody that wants to use one.
Some work better than others as explained above due to the resistances of the metals that the power has to go through to get to the atomizer however... For me... Unless the voltage drop is really substantial, I just can't tell the difference in a mech mod that a store says is "hard hitting" compared to a mech mod that is supposedly not so hard hitting and when push comes to shove and I looked up reviews of the devices online I see that the VD (voltage drop) is nearly non existent in my opinion. One of them had a VD of something like .1, and the other a .2. I guess I just don't have a refined enough pallet to tell the difference between a Hard hitting device that only has a .1 VD compared to a not so hard hitting device that might have a .2 VD. However, I'm sure that some folks do, or at least believe that they do.
I think of the VD as being sort of like computers for me. I'm a computer nerd and when I hear that some SSD hard drive performs 2.3% faster in the read/write abilities, I usually would go out and buy it (if I had the extra money) and to me I notice that it's faster, at least I *think* I notice that it's faster, it could also be the placebo effect of a new SSD installed in my computer with real world test data to show that it performs 2.3% faster than what I had previous. So, when I see people that say they can tell the difference and that it's major between something like .1 and .2 in VD, I have to believe them since I believe that I can tell the difference between hard drives that perform as little as 2.3% faster or at least I believe that I think I can tell the difference in real world use. LOL.
Anyway, the whole thing to me wraps up in this way...
How much does it cost
How aesthetically pleasing is the device to my eye
How smooth are the threads and firing button and lastly but most importantly
Is it safe to use or are there a lot of people saying that the firing button sticks and or it has some sort of overheating issues from somehow shorting out.
But they basically all work the same, delivering power to your atomizer, some do it more efficiently than others but all basically the same principles, just some are prettier than others and have a higher quality level than others..
Edit: So, do they perform drastically different? In my opinion no. They all perform the same as far as delivering power to the atomizer to turn our liquid into a vapor that we inhale. The only difference to me is that they may look different, and the quality of the materials used and how they're assembled is different on some mech mods but the vast majority that I've looked at or had are all basically the same, a tube (box or other shaped battery holder), that holds a battery and allows that batteries power to get to my atomizer.
