These are generally very easy to use. All of the buckle carriers that I stock here at Snugglebugs with the exception of the JPMBB / Love Radius carriers do not require an insert.
Personally I love a buckle carrier for a newborn that is fully adjustable and made from wrap fabric. Buckle carriers are super simple to use and really handy in that literally two or three buckles clicked and you are ready to go.
If you would like to take at look at the Kokadi Flip, Fidella Fusion, Mamaruga, Little Frog and Ergo that I have in stock you will see some really great carriers. SSC for newborns are more expensive than other types of slings. A good supportive ssc that will provide the proper support for your newborns hips and spine are priced somewhere between €99-150. They can be used for both front and back carries
If you are looking for a buckle carrier for a child 4-6 months plus the Kibi, and the Little Frog would be really good options.