I am very late to this thread but just want to say that I've been absolutely delighted with the Jane Matrix Light 2 which I bought when DD was born. We do long motorway journeys every 4–6 weeks and the peace of mind that DD was safe lying flat made these journeys easier.
The only thing I would say about the Matrix is that in the upright mode the recline isn't great so one we'd finished using the lie flat mode (DD is small so was still lying flat at 14m) I went on to buy an ERF to rear face up to 4.
I believe that the stats were that in 2015 fifteen babies died from asphyxiation from being in a traditional infant carrier for too long which was higher than the number of babies under a year killed in collisions.
It's an issue which has not had enough press.
Having said that, I would like more research into how lie flats behave on impact. The Which? testing painted a pretty good picture of the Matrix.
The Kiddy lie flat is also a rear facing seat so that probably wins on all counts?
Just a word of caution. Many babies don’t like being flat. DS2 had reflux and hated even a regular car seat as he preferred to be upright. I had to do long journeys with mine when they were little. Ds1 was fine in a standard rear facing seat . DS2 cried in the same seat every time after 2 mins and was inconsolable flat in Moses basket or bed. It is really common!
Which doesn't test car seat safety, they use the ADAC testing and then add their own opinion on things like fabric.
With regards to the number who die in a crash, that's hugely reduced by the fact that most babies are transported in safe infant carriers. Physics tells us that travelling sideways in a car can not be as safe as travelling rear facing.
My youngest was born in the EU and we were constantly travelling back to the U.K. You have to take breaks regularly anyway so I'd rather choose the car seat that fits in my car the best in order to ensure the safest position.
I had the Jane Matrix light 2 (nothing light anout it!) My babies both seemed to love it and I love my other Jane products (pushchair etc) but I could not recommend the Matrix as it drove me mad each and every day with its stupid twisty straps.