I totally stripped and cleaned my rear disks on 2012 Jazz and replaced pads with EBS (with a wear coating which cleaned the old disks). Since then handbrake performance has been superb.
Lack of skilled maintenance is a killer - as is faulty assembly - which is easy. The Cross in the caliper MUST be adjusted so the cross runs vertical/horizontal. And deviation and the pads do not seat properly and braking performance is poor and the rear pads wear unevenly. This is covered in the official workshop manual online .
It is not highlighted but I have seen some terrible examples on FB Jazz Owners Club.
Perhaps there's some difference with your 2012 model compared to the 2002~2008 range?
You wouldn't get the caliper to seat unless you align the cross on the piston with the pin on the pads in my experience. Was first caught out with that, on a KIA Picanto, many moons ago!
Braking performance wasn't an issue, as far as I could tell but handbrake performance pretty poor.