The opposite of that. Using somebody else's code, written for a different app in a different environment, is where most unexpected behavior comes from.
Here's the bits that aren't robust enough: any open source that isn't either used by thousands, or used in exactly the way you will be using it.
I would still be mostly using a 3rd part application framework if I rewrote some bit of it that I found problems with. I wouldn't be rewriting the whole lot - that would take forever. So I'd be left with a 95% of the framework being written by the 3rd party and 5% written by my team.
That 95% would still need to interact with my new 5%, and as it was never designed to do that, it could easily introduce new unexpected behaviour. Imagine trying to rip out (for example) the indexing code out of a database because it's performing badly and then rewriting it.
Imagine having control over the behavior of your code! That's the advantage of writing it. And I thought we were talking about replacing open source packages, not doing open-heart surgery on a faulty package?
Why would you think that? I explicitly said "Apart from the OS (which isn't where the problem lies), there's no open source in it."
It's a 3rd party commercial platform for developing your own applications on. To rewrite individual bits that aren't working efficiently would be a nightmare.
As for imagining having full control over the behaviour of my code - I had that quarter of a century ago or more when I started coding, and spent weeks or months building windowing systems and low-level networking code in assembler that I could achieve in 30 seconds with any off the shelf language these days.
Yes, my code was extremely efficient (it had to be, trying to do graphical comms software on 8086 machines didn't give you much of an option to do otherwise), but the use of my time wasn't.
The third party product that we're using is the result of many years of a large team of dedicated developers. Rewriting that from scratch in order to make sure we've got full knowledge of what's going on is an utter ludicrous idea.
Here's the bits that aren't robust enough: any open source that isn't either used by thousands, or used in exactly the way you will be using it.