I mean, with this custom thing, you have that question as well with downside is you cannot pick up the knowledge from off the street.
I became the Kafka guy at my current company, it took me about a week of reading and every time I had further question, I didn't have to bother anyone, I could Google and get data I needed.
When it's some NIH thing, you have to bother coworkers and knowledge is whatever is in YOUR company knowledge base with no ability to get knowledge from outside the company.
EDIT: You could also leverage contractors or outside support if not homegrown software.
Yup, and every time it breaks, you've got to pester someone whose job probably isn't maintaining that thing actively.
I worked at a startup with massive NIH syndrome, once. We even used our own in-house programming language, because it was "better than anything else out there on the market." It did have a lot of nifty features that others don't have: a pretty novel type system, programmatic macros, a built-in build system and other fun bells and whistles -- but also not-so-fun ones like having no syntax highlighter, LSP, or debugger, and having to constantly shuffle around your code to avoid ICEs in the compiler.
The compiler wasn't the product, but we found ourselves fighting that thing more actively than any of the real problems our custom programming language was supposed to solve. The CTO found himself spending all his nights and weekends mostly trying to get the compiler to not explode.
A few years later, after I had long left (for that reason, among many) I heard they switched to Python. Can't imagine how long it took them to get that all rewritten.