Just a note about something I picked up - Paed Physio warned against bubs sleeping in capsule for too long as the angle is bad for head/neck. OK if they fall asleep in car obviously, but if you have a chance to transfer them to cot/pram when you get somewhere, then you should aim to do that rather than leave them to sleep for long periods in capsule.
I've heard something very similar. It too was a warning not to leave kids sleeping in them bacause there was something to do with the angle of their head and it squashing their chest. So they were not breathing deep enough or something and therefor they were receiving lack of oxygen. I think!!! Can't be certain as I didn't take too much notice as I don't use a capsule.
As PP said, this was not about using them in the car, it was about bringing the baby in asleep and letting them continue sleeping in the capsule inside for too long.
Pretty sure it was a warning that was on the news one night!
I personally didn't use a capsule for any of my kids (no real reason why not) but have heard many people rave about them!