Have to agree on the Lipo mods, I have had many lipo's die prematurely
Another disadvantage with most lipos is battery capacity. The Hadron for example, only has a four cell 1600mAh lipo. My DJI quadcopters uses a four cell 5200mAh lipo. That would give decent battery life, but they are as large as four 21700 cells anyway. Plus they are $150 each. Ouch!
ShamrockPat also has a good point! Dual 21700 mods are a Big Brick, a quad 18650 would be ridiculous!
I don't know, I have some tiny dual cell mods. Like my HCigar VT75D is smaller than many of my single cell mods. And the Odin for example, isn't too big for me. They even fit nicely in my cargo jeans pockets. And the three cell Triade mods are even smaller than the Odin.
If you need the 400 watts? The Lipo's have way higher "C" ratings than 18650's
Well it isn't so much for having 400 watts, but more so for higher battery capacity. I mostly vape 15 to 25 watts. Although for larger builds, I have vaped upwards of 120 watts. And for some coil types, having a high wattage boost could be very useful. But say goodbye to battery life when you use it that high.