My first few bonding bags were fleece on the inside and cotton on the outside, they held up great. However, I live in florida and even though it's only one layer of fleece they still get pretty warm.
Then I relearned how to sew and made some with flannel on the inside and cotton on the outside, I love them they are soft and light weight and much cooler than my others with fleece.
You are correct they do need to be sewn properly or they will just fray and fall apart so they are a little more time consuming to make. If you use the top stitch method make sure the stitches are short, tight and don't have any loops. Some sewing vendors use a toothpick to test the stitches
There are a few awesome step by step videos on youtube I always use when making bonding bags.
https://www.youtube.com/watch?v=5JPrT9GWt04&t=16s
https://www.youtube.com/watch?v=rqS0ZrFlmwA&t=27s
https://www.youtube.com/watch?v=JqUEdYK7Do0
and
https://www.youtube.com/watch?v=BL_pruaJMlU&list=PLgM4VT26BX9GXd2NWWQGTtwOkloc_SoLb