Will an assembly part suggest that they shouldn't do this kind of business anymore, leaving me high-and-dry?
Exactly. And not only that.. what if you owned a business and you had an important weekend event which coincided with an assembly.. your staff wouldn't show up.
On the other hand, I have to say that at the restaurant I used to manage, we once made a deal with a witness family. We would cater their bridal shower, and they would steam clean all our carpets. They did an excellent job.
We once had a JW employed there who lied and cheated her way out of everything. After only a few shifts she stopped showing up and accused me of not scheduling her anymore.
In the end, I don't think that we can label the witnesses in general as reliable or unreliable. There are good workers among them and bad ones, just like there are good and bad ones among non JW.