I have received my Goblin mini V3 few days ago without the capacity extension kit (the seller owe me one and has promised to supplement it in the future).
The biggest CONS of this tank is its JFC:
1. Though there isn't leakage at all for this tank during vaping, but closing JFC does not prevent little juice leakage while refilling. In the case of Augvape Merlin, its juice flow control is working so efficiently in preventing refill juice leakage (through the AFC)
2. Only by comparison that I realize how much better the clear visual of JFC in Merlin. For Merlin tank, I know if I have accidently closed my juice flow at a glance, but this couldn't be the case for Goblin V3, I have to practically turn the glass tank in order to ensure my JFC is closed/opened.
However there is one advantage of Globlin V3's JFC in comparison to the Merlin's:
Unlike the Merlin tank, Goblin V3 tank does not have the problem of big bubble stuck at juice hole.
Because my Merlin tank has round juice hole, may be the newer ver which has squre juice hole does not have bubble stuck problem... may be...
Don't understand your criticism of the Goblin V3's JFC, there is a dial above the AFC lever, it has 3 liquid "icon" indicators showing you the open/closed position.
My issue so far is intermittent leaking through the airflow slots. All I can think to blame so far on my first rayon build is the wicking. Problem is, it will vape fine and be leak free for a few days, then put it down and next day fair amount of juice on the sides and underneath atty. Hard to pinpoint a problem when it doesn't happen all the time!