I was once a kayfun fan, have at least one of each variation of kayfun/Russian and sadly are setting nice and clean in the drawer of vaping history of ole, with exception of the new kayfun 4 which IMO is over engineered and has too many parts not to mention pricy.
But then came the day that a little known obscure company called Eleaf came along and designed a rebuildable tank that was initially designed to fit hybrid mode on the 23mm Evic Supreme in which I had at the time, although with the 510 adapter would work on any 510 device.
Little did I know at the time just how much of my juice flavor I was missing, until that glorious day I savored my wonderfull Wakonda from a Lemo, even tough I had been vaping my favorite juice for quite some time, it was the first time I ever got to taste just how awesome my favorite juices really were.
Haven't touched a kayfun/russian but have tried other rebuildable tank since, and you'd be hard pressed to try and convence me that any $50-$200 rebuildable tank could ever come close to my simple little $40 Lemo's.
But alas it's your money and your call, but I'll tell you now you cant go wrong with a Lemo period.