It's more than an aesthetic & workflow preferences. It's also a client management risk. Clients who have no development experience are often used to the same level of productivity to create the initial feature set. As development slows due to maintenance & performance issues, the client does not understand why. Clients also will use analogies, such as a mechanic overcharging for work & using words the client does not understand to justify high fees & billable hours...without understanding that changing a complex system with many undocumented & haphazardly implemented parts can create bugs.
Enter a maintenance project at your own risk. Due diligence of the business, codebase, and client's temperment, understanding of software development, decision making, & payment schedule can save you from headaches over the next few months.