My current baby had not been one that would calmly fall asleep in a carrier. Everytime I've tried to harness her with other front belted carriers it was combat to get her in and also combat to keep her in as she would try any form of gymnastics to wriggle herself out. Due to my own health issues, the belt often gave me really bad backaches too.
Since trying the Lenny lamb onbuhimo, she has fallen asleep numerous times in a carrier. She finds it fun to superman swing on to my back too, which is great not having to do gymnastics to get her in. It took abit to adjust with the weight being on the back than the front but definitely rate the onbuhimo!