The reasons are:
1. A plugin may require a third party DLL that is not present on the system.
2. These third party DLLs may require a hardware driver that must also be installed.
3. These third party DLLs may also require system DLLs that were not installed at the time the OS was installed. Or the DLLs may have been removed if the user "stripped" down the OS. A common one is the Printing subsystem.
4. A plugin may not be able to "find" these required components (even if they are installed on the OS) unless they are put in the PATH environment variable.
In all of these cases, the plugin will be reported as "defective". All that message really means is the plugin could not load due to some missing requirement. Perhaps the message could be worded a little differently.
In the case of the Contec cards, the correct PCI driver needs to be installed. When installing any plugin, the user must ensure that the hardware and associated drivers and software (API libraries) are there for the correct operation of the device. Most of the time, there are tools that come with the hardware to verify the correct operation.
https://www.contec.com/download/donwload-list/?itemid=8f8212f8-2dc1-4605-afd4-2d4867e50873#softwareSteve