FSHD is a condition that affects everyone differently. Some may be very severely affected, perhaps even requiring support for eating and breathing. While others may barely notice any weakness at all.
At the moment it is very difficult to say conclusively how FSHD will effect someone based on their genetics. Although it is known that people with fewer repeats tend to have more severe illness. This is not true for everyone. As diagnostics become better and better we should be able to start to piece this together and remove some of the uncertainty in this condition.