It sounds like you've found the unusual ones, Cielo - I know that the GreenCigs, GreenSmoke, LeCig and Nhaler ones have a different construction from KR808s. (The Nhaler ones are KR808s, I know, but they're somehow a new design I've heard...?)
The standard blank KR808d-1 cartos I've tried - from HealthCabin, Mist-Stix, GoodProphets, Heaven Gifts, Electronic Ash, etc. etc. - have all performed well for me, and all identically.
Sometimes I'll get a bad one that doesn't last more than one or two refills or seems to have some kind of structural defect. I've never gotten a "bad batch" from a supplier, though.
Filling, once you get the hang of it, gets very easy. Avoid the central tube, drip along the wall, keep the end-cap and plug in, use Taryn's spin trick.
I could not stand prefilled ones. All of them, even from The Mighty V4L, give me a nasty tangy undertaste. I don't know if it's the juice or the cartos, but it's like licking old lemon-scented disinfectant off a steel surface.