I am almost convinced my 50mm EDF motor that I bought is crap. I went with one off amazon and while it worked, it really felt lacking in power, even on 4s. I've had it in three planes now, and all three flew, but needed max throttle to do it, and really struggled to gain airspeed if they lost it. It currently is retired in my starscream F-22 (my user photo) which had a new but still unpainted front end on it. and basically is now a shelf decoration.
I had more luck in my F-22 than the L-39 but that may be because it's a broader winged plane so it glides better.
The L39 is a fast plane, no doubt, and you absolutely need to be careful with your inputs when you go quick. It lacks a rudder so it's a bank and yank and since there's no prop wash over the controls you need to remember to actually keep the speed up or you'll lose control. and stalling in a turn is what totaled mine. I'm hoping going up in size and adding some under-cambered wing tips will help with the power to weight ratio a little bit too.
I really liked it, which is why I'm trying it again up-scaled. I suggest you add some support for the battery along the nose section, I put a paint stick in mine, otherwise the battery weight will be more likely to rip the whole front off in a hard landing. and be prepared to re-build the nose a few times. I have yet to fly a EDF and NOT need to rebuild the front section at least once!
lastly, it really does need a good launch to fly, it won't really gain altitude till it first gains speed, and it wants to stall if you don't give'm a good throw! most of my prop planes just take a gentle toss but I found my first L39 really needed oomph in the toss to get it moving... otherwise it just stalled into the ground (like a glide) before getting enough speed to pull up.