It's _very_ easy to write C that is impossible to port to other platforms or other compilers. The reason is that more than for most other ecosystems, there are many different implementations with their own implementations and quirks.

It's also easy to write C that won't build on the system it was originally developed on, a few years down the line. But that is true for almost every other language ecosystem. I would say, C makes it _possible_ to write code in such a way that it works on many different platforms and compilers, and/or can continue to work in decades to come with relatively little maintenance.

The latter isn't true for most other ecosystems, because to get anything done in them, you need to rely more on that language and its ecosystem -- specific language features, specific libraries. C isn't a glue code language. It's very good at letting you create your own thing.

To do it well though, requires a lot of care and expertise.