IneedAbetterNickname, they aren't as easy to fit (which is one of the reasons they don't get a great review in WHICH), and they can take up more room. Then again, some of them are more compact than others - the Brio Zento being one of the smaller ones. More ISOFIX RF seats are becoming available as well, which greatly improves the ease of use.
As to why the government, etc, aren't doing more to promote them, I haven't got a clue but I do find it very worrying and extremely sad. ANEC (who do most of the actual tests that consumer groups such as WHICH base their reports on) consider that children kept in RF seats through till about 4 yrs of age have far few head, neck and chest injuries than their forward facing counterparts. I don't know of any research that has found FF seats to be safer than RF seats, but there are studies going back to the 1960's that find RF seats safer than their FF counterparts - this isn't cutting edge, new information . In Sweden, where there is an additional test car seats have to pass, you can't buy a FF Grp 1 seat as they don't pass the test - the main difference being that the Swede's have a maximum acceptable force on the child's neck in a forward facing accident (something the ECE test doesn't do).
What's even more bemusing is why British manufacturers that make RF Grp 1 seats, and promote them in Scandinavia, and (in Scandinavia) make a big thing about how much safer it is for young children to travel RF, won't even allow UK retailers to buy their RF seats from their UK production facilities. Apparently "UK parents prefer the colour ranges on the FF seats". Because that's a totally insurmountable problem, of course
As for big retailers, fundamentaly their purpose is to make money NOT to promote child safety. You'll often be told that "RF seats aren't legal in the UK as the UK has additional tests that the RF seats haven't passed". Fire safety wrt the covers is often brought up. All of this is false - the UK has no additional requirement over the ECE tests. To succesfully market RF Grp 1 seats in an environment where there is no government indication they're even legal and where "everyone" puts their 9 month old (or younger) in a FF seat would cost a significant amount of money. What's the incentive for them, when parents are already buying car seats from them?
Anyhow, getting back to the question in hand, if I were looking at a FF Grp 1 seat I'd look very closely at the Kiddy Infinity-Pro. It's a German seat which doesn't use a 5 point harness - there's a thick bolster cushion that goes across the child's abdomen and the shoulder's are unrestrained. As the shoulder's aren't tied to the back of the seat, if the car IS in a heavy frontal impact the entire upper spine can bend to take the strain (which is a big improvement over all that energy having to be accomodated by the neck). I'd be very interested to see how they got on in the Swedish tests! I've never seen one in the flesh and I'd have a lot of questions, but it would be a the top of my list of seats to consider