Same issue here. Got my first brew in there and it's slightly leaking at 20PSI. It's so difficult to get a grip of anything to tighten it further - and it already is tight. I got a good deal during a water pressure test earlier but as you say, you don't know if you've got it right until it's too late.
Received the replacement today. Initially I filled the Fermzilla with a couple gallons of warm water. Added the collection cup (valve closed) and no leaking. Once I opened the valve it leaked (dripping) from the same place as the other. I only hand tightened the collection cup.
I removed the cup, added a thin layer of keg lube to see if that helps. Opened the valve and still drips. it's not much but more than i feel comfortable with. At this point I am no interested in using the strap to tighten the crap out of the cup to prevent leaking. If this thin doesnt seal properly with just hand tightening then I dont want it.
The idea behind the Fermzilla is great. But the sealing sucks. Seems like i am the only one having issues with this and maybe is just me being petty. But I cant have issues with the cup not sealing especially with trub removal or dry hopping and not know if the cup will seal until I open the valve.
Anyway, going to play with it a bit more tonight and if I cannot get it to seal to my satisfaction, I'll return it and go with the Kegmenter.