A defining characteristic of legacy systems is the high level of difficulty involved in further development or modification of their existing functionality — often combined with low performance and poor user ergonomics. This is the result of a complex interplay of multiple interrelated factors.
Low Code Quality
A long system history often means that the code has been developed over many years by different teams with varying skill levels — frequently using a patchwork approach, without deeper analysis or attention to logical consistency and optimization. This typically results in unacceptable performance degradation and makes diagnosing emerging issues significantly more difficult.
Complex Dependencies
Years of ad-hoc development often lead to a loss of modularity — the clear separation of functionality and relationships between components. Poorly documented and difficult-to-trace dependencies between different functional areas of the system often require an unpredictable amount of effort for analysis before any changes can be safely made (Impact Analysis). Simply put, it becomes difficult to foresee what hidden or indirect issues might surface across the application when modifying just one part of it.
Complex User Interface
Due to the difficulty of modifying the system, changes are often implemented by simply “patching on” additional elements — such as fields or buttons — without maintaining contextual consistency, naming conventions, or alignment with the application’s existing logic. In some cases, the lack of dedicated functionality leads to workarounds, such as entering critical data into comment fields or other areas not intended for that purpose. This not only reduces the application’s clarity and increases the risk of user error, but also creates challenges for reporting and integration with other systems.
Limited User Interface Accessibility
A common trait of older-generation software is a user interface that—even if browser-based—is typically static, with layout and content designed for a fixed screen resolution. This limits or severely hinders usability on modern mobile devices, which are now a standard part of the workplace. In some cases, even more restrictive architectures are still in use, such as so-called “thick clients” — dedicated applications installed and configured directly on user workstations — further reducing accessibility and flexibility.
Outdated or Improperly Applied Technology
Some older technologies lack — or implement inefficiently — critical mechanisms needed to ensure the high quality of developed applications. A typical example is the absence of automatic data consistency controls (commonly referred to as database constraints), which means that any code modification carries a significant risk of introducing errors. Such errors can lead to subtle and hard-to-detect disruptions in data logic, which in turn may result in serious and unpredictable business threats — such as malfunctioning services or billing errors affecting customers.
Low Availability of Specialists
Due to the age of the technologies used and the long lifespan of such systems, specialists with the necessary expertise are increasingly hard to find — and their services tend to be costly.