Most parents seem to find the rigid Isofix system with a support leg far easier to use than LATCH. When LATCH was introduced there were unfortunately many compromises made.
Isofix seats for the ECE R44 standard have maximum rear facing weight of 18 kg (40 lbs). The car seats themselves can weigh 15 kg. The new R129 European standard, also called "i-Size" is looking more at height of the child than maximum weight.
There are discussions of raising the Isofix limit to 23 kg which would not be a problem strength wise. The margin of error regarding strength of the Isofix connectors is huge. But working together with car manufacturers is very time consuming and they don't want any changes. Except Volvo.
We can see this in the implementation of Isofix. Since 2013 all new cars must have Isofix. Some manufacturers, like VW starting adding this voluntarily in 1997. Thank you VW....!! It's been 25 years from beginning of Isofix to complete implementation.......
According to R129 seat plus child can weigh a total of 33 kg (73 lbs). There aren't many new R129 seats yet but these are typically as heavy as previous Isofix seats. They weigh around 15 kg which means maximum rear facing weight is 18 kg.
There is one great exception and that is the brand new seat Concorde Reverso. It's a really strong and well designed seat using different construction than before. The seat basically consist of a strong aluminum frame with EPP and EPS foam around it. The construction sounds weak but it's very strong, I have personally seen it being abused on the crash rig.
The design is great but the main feature is the low weight of the seat. It only weigh 10 kg (22 lbs) which is unheard of for an Isofix seat. That means that the maximum rear facing weight limit is 23 kg (51 lbs). The seat can be used from birth to around four years of age.
In my personal opinion you don't want to use a Cybex Sirona for many reason. The seat will last rear facing to 18 or perhaps 24 months which is a very short time. At least with our Swedish standard of rear facing to age four or longer. It must then be used forward facing with the shield system, or "impact cushion" as it's also called. A child should never be placed in a forward facing shield system.
Sitting with a huge pillow in the face is warm and uncomfortable but there are also serious safety issues with huge neck loads, submarining, huge abdominal pressure, chest deflation and rollover ejection.
The FF shield systems, not only this seat, is kind of using a loophole in the ECE R44 standard since abdominal pressure isn't measured. We can also see from ADAC testing that the neck loads aren't low, they are sky high.
Crash test dummys are getting very advanced, and obscenely expensive, and there is now the possibility of abdominal sensors. We are trying to add this to the standard so that shield systems can't be used but this is a long process.
