Professional compiler engineer here, C is a mediocre intermediate language.

Let's start with an excellent quote from Wittgenstein. "The limits of my language mean the limits of my world."

Using C as your intermediate language means that your expressiveness is limited to valid C programs. This is workable but only if your language can be mapped to C in _useful_ ways.

For example, let's say your language has behavior similar to scheme's tail-call. How would you get this behavior from a C compiler? You will never be able to make this reliably across optimization levels, etc.

Guaranteed tail-calls are the tip of the iceberg, there are a lot more features which cannot be reasonably mapped onto C.

Real compiler IRs increase your expressivity beyond what the C language designers decided was important.