Graphology can be very reliable or very unreliable, and it has nothing to do with the fact that the same person writes in different style. On the contrary, the most difficult analysis is of "frozen' types of handwriting's that do not show any change in anything whatsoever.
It is much more correct to say that there graphologist that are reliable and other that are not.
How can we know which is which?
1. Check his report against real facts in the field.
2. The reliable one can easily say "Sorry, i have no idea about what is going on with this case".  The non-reliable one, knows everything about anything.
It's the same with physicians, believe it or not and for all other professions, come to think of.