I hear you 100% and wish I had an answer. When I slap a carto in there that's terrible it really ruins the experience. I put juice in until its at the top of the filler starting to pool, let it sit 20+ minutes, top it off again if necessary, then punch it and put it in a tank. I take 5-10 long and hard dry hits and then start firing it with low voltage and slowly work it up to my normal ~4.5-5.5 volts on a 3ohm carto. Even with all of that, doing exactly the same thing with the same exact juice, my experience is all over the board and that's BS IMO.
I should be able to have the same experience each time and there is no excuses for that. When they work right, they work great and I love the flavor, but when they don't it really just kills it. I've only had one that was DOA, but more than 2/3's have had serious flavor issues.
The Vivi Nova 2.5 I got works pretty well, but I'd say the flavor is a little muted and the draw isn't as satisfying as a good working carto IMO. BUT, and this is a large but, it seems to be much more reliable in it's performance, it's cheap, and very easy to use. You can even re wick/coil it, though I don't have experience with that as I'm still on my first of 3 provided atomizers. There is one other inconsistency I found, I used the atomizer labeled 2.4 ohm, connected it to the ProVari and it reads 2.8 ohms. Not a problem for my ProVari, that's in the sweet spot for a ProVari, but it's not what it is labeled at. People using it on a Twist or other devices that can't read (or effectively read) ohms on load wouldn't know this and might be running it at an improper wattage for their juice. I've also read of issues with the connector protruding down too far, however mine is perfectly level.