care to educate ME?
Sure, I'll do my best!
Harness snugness is extremely important when using a car seat. Using a blanket that goes behind the baby, snowsuit, or bulky coat can prevent you from getting the harness tight enough. Those things all compress in a crash, meaning that they might feel tight before they're compressed, but once compressed, you end up with a bunch of slack in the harness. As someone said above, there might even be enough slack that the child is ejected during impact.
When it comes to those blankets that interfere with the harness, the problem isn't just bulkiness. Do the holes match up exactly, perfectly right? Was it crash tested with your seat? Probably not, to both of those. So, the harness might not be sitting where it's supposed to sit. The material might bunch up during a crash, introducing slack. And, since it probably hasn't been tested, your child will be the test example. I personally don't want to use a seat in a way that hasn't been tested. That's why I use the proper recline, don't combine LATCH and seat belt, etc. If my seat says that I must use the base, I'm not going to install it without the base. This should be just as clear a safety issue. It hasn't been tested with a Bundle Me, so I'm not going to use a Bundle Me.
Now, blankets that go OVER the baby are a different story, since they don't interfere at all with the function of the harness. There are many safe alternatives to a Bundle Me. They are referred to as "shower-cap" style, since they are fabric with elastic all around the edges. Land's End has one, if you want a "big" name, but there are others available on the internet.
This is a video someone posted here recently, showing how winter bulkiness is a problem in infant seats, plus it lists some alternatives.
This thread talks about the pinch test. Try the coat, bunting, etc., and tighten the harness properly. Then, without adjusting the harness at all, remove your child and the coat or bunting. Put your child back in and do the pinch test. If it no longer passes the pinch test, the coat or bunting should not be used in the car seat.
Here's
another thread that was posted here, and
another.
This one in particular talks about crash testing, mentions how the harness doesn't line up properly -- it was 3 inches off! Meaning that it isn't just a compression/puffiness problem -- there's a link to a photo in post #15.
Then there's the
car seat manufacturer's statements. They warn that using any aftermarket products voids the warranty. Granted, that might not mean much after an accident, but still. They feel strongly enough about it to make a statement about it.
Thin fleece jackets are nice and warm for the car. There are photos in that video I linked to. I keep Toby in a fleece jacket inside, and take his parka along. I put the parka on when we get outside. He was 5 months old when I started doing this, because he was already in a convertible by then. Yes, it's a hassle, but I know he's SAFE. That's the most important thing, and trumps convenience!