i LOVE my Nomad, and i also loved the Matrix. the orange button did slacken off quickly and i found it a breeze.
i wish i could remember the exact details now but i did investigate the lie-flat issue at the time and i decided to go with it. as far as i was aware Jane said that the tests had not complied with their own recommendations for weight because the article had to be the same across the board.
i just don't see how the seat could roll, really, it's tightly attached to the axle of the car, not reliant on a stop-mechanism on a seatbelt (which has always seemed supremely dodgy to me after i saw a consumer programme that tested the mechanisms of seatbelts in black cabs and found that half of them didn't work in an actual crash situation).
just looking at the baby in the lie-flat, i can't see how that massive tight belt that covers their entire abdominal region isn't as safe as a five-point harness. but i'm not a safety expert, obviously. when DD was tiny i hated seeing her in a sit-up carseat as she just looked so uncomfortable. also it doesn't take long for breathing to become compromised in that position.
the one thing i would say is that you should have the matrix fitted by a jane-trained person, because Jane insist that you twist the orange clip things as you attach them and that seems counter-intuitive so you need someone to show you.
i also marked the area on the belt where the clip goes in pen. i wanted to see if the belt would stretch at all or the clip move. the clips did move after a while, so i made sure that i tightened them every single time. it was easy to do, though, so not a problem.
i suppose that any seat is dangerous if incorrectly fitted and the seatbelts not kept tight, so the main thing is to keep an eye on that regardless of whichever seat you choose.
good luck, i really think my Nomad has been an excellent purchase. And it still looks great after a year...