Service X7 rarely stops at Grangetown - passengers tend to let it go past as they're still unfamiliar with what the service is (and it's been around for quite a while now). I have no doubt that the same will apply for the X21 too.The X21 looks to stop at all stops between Ryhope Village, Stockton Road and Park Lane according to the timetable on their website.
Can't remember off the top of my head where in Ryhope the X21 stops now, but assuming it's near the War Memorial...
Obviously with 16 buses per hour through Ryhope (6x 29/39, 6x 60, 4x 61) from Go North East, it's unlikely that the X21 would be the first one to turn up anyway.
Again why are the times for the X21 to Sunderland below Newton Aycliffe Awful website, just awful!
I assume people will let the X7 go past as people will see the X and see it as an express service so will assume it doesn't stop at that stop, just hope things aren't the same for the X21.