As someone that writes sophisticated scanning software that has to support many different scanners and hide the driver’s UI it is my general experience that the writers of TWAIN drivers do not bother to implement most of the capability querying in the TWAIN specification. ISIS drivers do.
This is a real pain as we have to maintain a database of scanners and all their individual features and supported settings.
TWAIN drivers also do not necessarily adhere to the specs. for setting features either. For example DPI might be a range or an enumeration. Brightness might adhere to the 1000 based range or use the ISIS range based upon 127 and, as I said the getter methods to ascertain this may not be implemented.
If you know these features or have the freedom to allow the user access to the drivers UI then the drivers are typically fine for scanning etc.
Just my personal experience.