Who tells their clients to come over to their garage anyway?
You would always say come to my studio...
upon arriving and seeing that it is a garage, if they have a problem or are uncomfortable then end the session.
As long as its neat, clean, well ventilated and the temp is controlled, i wouldn't be uncomfortable in a garage setting as long as it seemed legit and professional.
Again, its all up to the personability of the photog.
And besides, garages have the uniquenes of being able to be changed on a whim...
not that you can't do that in a living room/in house setup, but i would think garages as being not totally finished rooms have an advantage over "finished" rooms.