From the other side of the fence, more often than not when someone offers an open source or COTS replacement to an in-house system the result is that it replaces easily 90% of the functionality, but for the last 10% you need to either serious mental gymnastics to fit your system into the third party system's way of doing things or just put more effort into patching it than doing it from scratch would have needed. Sometimes the 10% is not really needed, sometimes it's the whole reason why the product was built.