There are issues with valuation when the existing customer base starts to slowly leave and the business is not as profitable as it was on paper. That is why never sell the business as "Turnkey".
I have seen insurance policies get voided when ownership changes and the new owners have to be up to date with the customers insurance requirements. They cannot be grandfathered in on old terms from years ago because the owner was buddies with the customer at the time.
I have seen companies get sold and keep the name just for the customer list. It has worked, but can be very risky. Customers might not want to see a change in ownership and if you do it on the down low and dont tell them, they can get even more pissed off. This is not to say this happens to every business, there is always some level of risk involved. Especially when coming up with an evaluation on assets.
Are the customers an asset? Whats stopping them from going elsewhere now? Does the company make a certain product that cant be easily reproduced by someone else?
Everything in business is uncertain, machines break, employees quit and customers leave. If every aspect of the business were to be looked at like this then there would be nothing of any value for a buyer.