My guess is it's mostly user error
by folks new to vaping or not finding out how to use it prior to purchase.
Example...Experience tells me that if I leave any device like this(basically a clearo) in a high heat environment or in the sun, IT WILL LEAK, it's the nature of them. A newb blames it on QC or a bad design.
I am the newb who reported that. It was basically pouring the tank empty in just a couple of minutes in the sun. The eGrip loaded with an Aspire BVC head and the Subtank Nano with a rebuilt Protank head were standing right next to it and did not leak at all. Also it's very sensitive to shock, I put the latter ones in my pocket any time, but I did not dare to do that with the eGo ONE.
I do not bash the product, too much I love the size. Even the tanks are tiny compared to other ones with a similar capacity - did you know Nano means huge

And the vape experience is pretty much exactely how I like it. Unfortunately that experience only lasts for a short period of time in between cleaning and the start of gurgling.
That's simply reporting my user experience. I got 3 tanks and went through 7 1Ohm heads so far. The results are pretty much the same. Even with 90% VG juices.
I am tight draw mouth vaping, not by error but choice!
UPDATE:
Good news. I've rewicked the head. I only replaced the inner
glass fibers with Fiber Freaks (Density 2), then rolled it up again with the stock cellulose-like outer wick. As the Fiber Freaks pads are a litte thicker than that 'Continuous filament glass fibers' - hey Joyetech, yes it produces good flavor, but nobody wants this crap breaking off! - I cut off a little of the outer wick to not increase the total density. The head is extremely easy to rebuild due the top lead of the coil held nicely in place by the heads top cap. I'm pretty happy with that as I've planned to rebuild the heads anyway.
I've only gone through one tank yet, but there is no more gurgling. I also put it in the sun, for at least twice the time I originally did. Again, no leaking.
Whether or not I could exactely match the total density of the wick, my guess is that those stock glass fibers have no wicking capabilities and thus are responsible for the leaking. Hopfully they aren't to health issues.
This is just a first impression, I'll report back once I've gone through more juice and I've finally carried it around in the pocket.