Shimano Front Freewheeling System ("FFS") was, like the label says, a system. It had a special crank with the freewheel in it, plus a rear thread-on gear cluster that didn't turn freely. This unfree-wheel component did however contain a stiff one-way clutch that would turn if for instance something became jammed in the chain.
All these parts are old and unsuited for index shifting. There's enough drag in the freewheeling chainring of a Bafang mid drive to cause unwanted behavior in the derailleur and chain if the rear freewheel doesn't turn freely, such as if you use a Shimano FFS gear cluster. The derailleur cage will be pulled forward, when coasting, and slack chain spooled over the top of the rear sprockets.
There's nothing to be gained by using an FFS cluster, and in most cases it's a very bad idea to use a thread-on freewheel with a mid drive, rather than using a rear cassette hub. Extra chain tension will worsen the chronic axle bending and breakage that originally caused cassettes to take the place of freewheels.