I don't really know about the limits, except my own... I have asthma, and for the last 20 yrs, I had to smoke ultra-lights, or end up spending more time coughing than smoking. Yeah I heard all that "well you should just quit," to which my reply was always 'well you should just mind your own business.'

But the cigarettes I smoked, though ultra-lights, still had a bit of throat hit, which at that point was my main criteria -- does it actually feel like I'm smoking a cigarette? rather than sucking air through a straw. Now, I have this great replacement, and my asthma is definitely improved a great deal... but not gone, and it never will be, and certain kinds of stimuli trigger it horribly. So I have to be careful how hot, how much TH, and how much and how thick the vapor is. Lower ohm coils are nearly always hotter, and nearly always produce a great deal more vapor -- hence my problems with my Caterpillar.
I finally managed to build a coil for that thing that I can actually stand, and as long as the juice I'm using in the dripper has ZERO throat hit of its own, I can use my caterpillar to taste new stuff, but it'll never be my primary, just too much fiddling around -- I want to fill a tank and not worry about it until it's either empty, or it's time for me to drop the nic a little, as I do at night -- but even that last bit is easy now that I have 2 terrifically-working kayfuns (clones) -- I have a clear M-tank for day, 9-10mg, and a smoke M-tank for night, 6-7mg, so I just switch between them.
I also discovered recently that VG, even as little as 33%, just flat out smothers me -- even my rescue inhaler won't work if I can't inhale enough to get the medicine in there. So I'm back to about 80%-85% PG, and don't walk around feeling I have something in my chest that needs to be coughed up, and not feeling smothered and completely unable to breathe. PG is indeed very dehydrating, and my swollen feet and ankles from that dehydration are painful and a bit worrisome, but being unable to breathe is far worse, so I'll just have to deal with the PG dehydration, including swollen feet and puffy ankles, as best I can.
Andria