Seatbelt and LATCH strap webbing are designed to stretch in a crash. So, say you have what you think is an acceptable installation with 1" or less of movement at the belt path of your child seat. Only you have a seat protector, that in the process of the crash, gets shuffled. Now your 1" or less of movement at the belt path has become 2" or more of movement on one side and/or both sides and/or front to back. Then your 2" or more of movement multiplies when the belt system you used for installation stretches. Imagine what your child seat, and your child IN your child seat, are doing on impact and afterwards with all that belt movement. It's pretty scary.
Seat protectors are bulky and rubbery and can interfere with proper installation, or the assumption that you have a proper installation.
They are not approved for use with child seats due to the fact that they are not crash tested with the child seat in that vehicle by the manufacturer(s). How it would respond in a crash is unpredictable. Maybe nothing would happen. Who knows. But unless you're comfortable enough to make one of those "parental decisions", which, essentially, leaves your child playing the role of a crash test dummy, you probably shouldn't use one.
I don't mean for that to sound as harsh as it probably does, but, ultimately, that's what it comes down to. It's just not approved for use with the child seat and, really, that's good enough reason for *me* not to use one.