I’ve heard the lay flat/ bassinet type car seats are great if you need them for medical reasons but do not perform as well in a crash vs your standard infant carrier x
We almost bought the Nuna mixx next with the cari next because we have a wedding to go to in April that's 4 hours away. We only didn't in the end because the sales lady couldn't tell us if it was overnight sleep safe like the standard Nuna one. We also heard some babies hate the carry cots so didn't want to risk that! But honestly, they sound a good idea for longer journeys! Although you're likely to have to stop every few hours anyway. The car seat we've got reclines well so is almost as good.. just not quite! There are a lot of lay flat options now, so doesn't need to be the carry cot option.
They say no longer than 30 minutes for the first 4 weeks and then 2 hours after that.
They can be in their car seats max 30mins - some people say they need a break after this and be taken out for 20min, others say 30mins is the max per day. Us and others have risked slightly longer journeys such as 40-50mins but there is a big risk in terms of babys breathing for long periods, hence the 30mins guide. If we have gone longer than 30mins we’ve made sure someone is sitting in the back keeping a close eye on baby and their breathing. Not sure about the lay flat car seats sorry - they’re quite new i think