I am very interested in this thread. I also purchased the Keylab 88 online from a trusted vendor back in June 2015. I had never played it in a store and went with the "Fatar" name, reduced weight (like you, I've used an Alesis, but mine is the original QS-8), and others' reviews at the time.
My experience with the Keylab 88 is very similar to yours, except that I decided to not return the unit, instead taking the approach that by playing it, I would build up endurance and strength, which has happened. But, in all honesty, the action is _very_ dense feeling, and (my guess is that) the aftertouch sensor may have a lot to do with it. I can "feel" that I am pressing into, and bouncing back from, something at the bottom of the key travel, and I believe the sensor (must be some sort of "strip") is the core - wish it could be removed, if so.
When I first played the action I was concerned about whether I could endure through a set of music, or even execute certain parts with the action.
Now I can, but I think I'd buy another 88 key controller in a minute if I found one that felt better, all bells, whistles, pads, buttons, etc. aside.
I do have to wonder about the demo unit you played - and why that unit felt so much better - a shame they all don't.