When new updates come out undocumented changes in the code breaks our implementation leading to days of work getting everything working again. Tech support knows NOTHING about the changes that are made in updates so they are completely worthless to turn to for help.
Very poor, practically non-existent documentation.
Cryptic (if any) error codes returned from the object.
No input validation in the library so strings that are too long cause the library to crash without any information about why it crashed.
No documentation showing the maximum string length that can be put into each field of the library which makes the above problem darned near impossible to solve. Review collected by and hosted on G2.com.