[folivier]

I'll agree with Richard, in my former life I've used literally thousands of fittings with ferrules (mostly stainless fittings and tubing) and had very few failures or leaks. An occasional re-tightening was necessary due to vibration. I do have an assortment of push-on DOT fittings mostly for on-the-side-of-the-road repairs but try to use ferrules where possible.
Just make sure you use a ferrule with the brass inner sleeve that will prevent crimping and cutting the nylon tubing when you tighten.
Hey, what brand ultrasonic leak detector are you using?
I usually use soap and look for bubbles but it's messy and hard to get into some spots.