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.