Not familiar with flycut, but had a look through their issues. There are a few security-related, and a bunch of iCloud stuff. Eg, this one to add the option to disable iCloud sync, which is still open after 4 years:
From that it seems clipbook may be better for people who care about local-only or have other security-related concerns (perhaps related to workplace policy, for example). Maybe someone who knows more could weigh in?
