The biggest problem with the jj cold bundle me is that it routes the harness straps funny... instead of coming out horizontally, the straps are forced to twist so they are vertical to go through the bundle me. The width of the fabric is too wide in the back to sit flat with the straps flat on the shoulders, so they either end up twisted, or not tightened up - or else baby ends up with a bunch of fabric bunched behind the back... definitely not comfortable.
In addition to that, in most seats it also results in the hip straps going up to come through the slits placed for them. So they go up to get through the hole, then back down to go over the hips and be buckled. This obviously puts a lot of bulk between the child and the strap and results in either a harness that is miles too loose, or a screaming baby who is horribly uncomfortable.
Then you have the padding factor on top of that... add them all together and you have a high likelihood that the straps are miles too loose and extra padding behind the baby that is likely to compress.. end result = greatly increased risk of baby being ejected from the seat. Not to mention the twisty straps that result from the straps being turned within the slots of the seat.
Custom carseat cover is usually an issue with slots not lining up perfectly.
If it helps any, you can tell her you know somebody who lives in Canada and went through a winter with a premature baby with just a popover shower cover despite -30 and -35 (celcius) temperatures. And sometimes he was sweaty when I got him out of his seat. Most people overdress their babies to begin with, and I know if I was a baby, I wouldn't want to be trapped inside a hot and sweaty bundle bag in stores and again in a warm car. Nobody in their right mind would cheerfully sit in a sleeping bag inside their house... babies don't like it either.
You didn't mention other after market products, but my approach to them is that they are generally added by parents to try and fix a problem. If you correct what the problem is, the need for the aftermarket product goes away. :thumbsup:
Often times just removing the bundle bag and buckling the baby in and tightening the harness up is enough for a parent to see the difference and keep the bundle me out. I've found in cases where parents are resistant to the idea, that that approach has always worked. Sometimes seeing is believing...
eta: Above and beyond the reasons mentioned, the aftermarket products weren't tested with the carseat and in many cases carseat manufacturers specifically prohibit their use. In choosing to use one, your friend is essentially choosing to waive the manufacturer of the carseat from all liability if her kid had something happen in a collision.
