Some reasons for not having a shiatsu session might be if someone is elderly and they have a hard time getting down onto the floor. Most practitioners can adapt their technique and work with someone in that situation either on a bed or on a massage table. So it would just be a different style of shiatsu in that case.
If someone is dealing with an injury it's just really important that the practitioner that they go to is aware of that and not necessarily works with the injury but works around the rest of the body to balance it so it doesn't aggravate it or make it worse.
Another precaution about when not to get a shiatsu session might be after a surgery, just giving the body time to heal on its own first before having hands on intervention. If someone is sick, has a cold or a flu, it's often better again to let the body rest before coming in for a session and interfering with the body's natural way of finding balance on it's own.