Ok, just to make things clear since this thread appears to be full of a lot of misconceptions and a lack of understanding of electrical circuits and ohm’s law (not a suggestion).
Series = double voltage, single cell mAh, single cell amps. This is a fact, I don’t care how you feel it may last longer than your other set ups, how you use that double voltage default, single cell mAh and amps is up to you but it’s a fact that this is what you get by pushing the voltage of one cell through the other. There is no contradictory perspective that will change this fact. Can you make your super low ohm build last longer with this than you can a single cell, boosted voltage circuit, yes, you of course can because the load is being taken off the cell/s and chip in doing the boost but it is still single cell mAh and amps.
Parallel = Single cell voltage, double mAh and double amps. Advantage here is more mAh and more amps to play with. Could you do over 75 watts with this set up? Yes, of course you could but you would be relegated to doing so on a mech because sadly no one makes chips specifically designed to boost voltage in this manner for vaping at this time. Is there an advantage to doing it this way with an existing 75watt or less board, yes! Because more mAh means even with a silly low ohm build, it will last longer.
Efficiency of the chip is a factor in all of this of course, every one of them has a middle ground for maximum efficiency at the highest watt setting. From here you have the most freedom to take your watts up or down, tailoring things to your preferred draw and flavor of the liquid being used. All chips have a maximum amp they can pull, as such, a minimum spec of battery that must be used with it (the constant amp output, not the darn pulse). Demands of this are based on the amp load of the ohm it can handle and the basic demands of the chip to do what it is doing. In that respect, whether series or parallel, the circuit has its amperage requirements when it comes to cell/s used with it.
How much you vape is the biggest factor in this as a chain vaper vs someone who doesn’t vape as often will see their cell life span be drastically shorter than the other because they don’t activate the circuit as often. You can try counting mls of juice burned through a day to measure this but unless someone does their set up exactly the same as you, from RBA used, draw, gauge of wire, ohm, device, liquid and watts setting, comparing one vaper’s battery life to another is utterly pointless.
The iStick 100 is a series device, not a parallel one, please don’t confuse that, anyone, nor let any “new” vaper pick one up, please. Personal educated choice is an informed risk taken by an individual in doing what they’re doing. Putting a series device for vaping, putting massive amp load, low ohm coil builds on them for that new vaper to hold up to their face with no knowledge of cell pairing, rotation or charging regime is just plain irresponsible and the dire potential results of doing so have caused personal injuries through the years of vaping thus far and seen at least one business I know be forced to close down due to a spurious law suit on the part of the injured party. This was back before regulated chips that will yes, protect the user in the event of many failures but all things can fail, including the protection against doing so and what can stop your cells from failing isn’t merely the brain in the device you have them in, it’s also how you use them and charge them that can cause a massive half life of their capacity, chemistry and stability.
Lastly, were it not for shoddy RBA designs with massive chambers, there wouldn’t be any experimentation of anything even approaching 100 watt going on in vaping but sadly too many RBA designers don’t understand physics or ohm’s law and as such, make stuff that is pretty externally and internally is just about the same as all other crap on the market bar different post designs that merely exploit the desires of instagram coil builders to expand their canvas horizons and opportunities to waste an abundance of kanthal but hey, they look pretty
