bcache is inherently designed to be an ssd cache that sits in front of slower bigger disks. Bcachefs is an extension of this into it's own filesystem. iirc the words of the bcache creator were: 'we've implemented 80% of a filesystem here, might as well go the rest of the way'. So how much it thrashes a disk is based on what position you give it in the architecture. The caching ssds are going to be used heavily, taking advantage of their fast random access to manage all random accesses, while sequential operations generally go to the slower disk that's set as the background device. The background disks will tend to be accessed less.

So yeah, it's based on what kind of disk and position in the bcache, and what caching options you enable. If you want to look into it further, bcache is fs agnostic, so if you can find some tests that have been done for bcache enabled for classic linux filesystems, like ext4 and xfs, that include hardware degradation info, you'll probably end up with similar usage and hardware wear with the actual bcachefs.