u/Efficient_Reward401

Hello, I'm very close to getting a new roof and I keep reading that you can't use ice and water shield over the entire roof because it doesn't breathe, but you can use synthetic underlayment on the entire roof? Does synthetic underlayment actually "breathe"? How much breathing does it really do with asphalt shingles installed over it? I guess I'm just confused as to why synthetic underlayment is okay to use on an entire roof but not ice and water shield (besides the steep price)? Thanks