One of the biggest problems of this is the tribal knowledge that develops around it. I worked at a studio that had something very similar to EASTL, but had joined the studio after an exodus of senior people.

It meant I had no idea how to use the custom libs. No documentation, no one left in the office to tell me how its used, no Stack Overflow to answer even trivial questions.

I left after less than a year. The studio closed down 2 months after I left.