Well I'm pretty sure of 2 things.
If babies were in danger of being in their car seats for more than however long, or there were a death rate increase because of this then I'm sure it would be all over the news and car seats would be under a change of design asap
Secondly....I hardly think mothercare of all the companies, are going to sell something that would endanger a baby's life! I'd imagine all car seats would come with a huge red safety warning sign if there were a problem.
I have an urban detour that I used with Harry and he used to sleep in it loads. It's a fab system!