I agree with everything you say however, if the two things you rely upon are, in their own words, such a mess then I have to imagine that mess consumes an equal if not more inordinate amount of time to integrate with.

I'm a software developer too. In my career I've had to implement logic to work around buggy libraries, spend time looking into bugs in others code... It seems that at some point that the means don't justify the end and having my own code base that I'm familiar with would be easier than fixing theirs and waiting for disputed parties to figure out what they are going to do.

And I never claimed doing your own was easy. All I'm suggesting is that at some point it's easier than dealing with what you got.