Any lingua franca will end up deviating in pronunciation from how it's written. For example, Finnish is said to have very shallow orthographic depth (the technical term for this phenomenon), but you can imagine if people from across the world began speaking and writing Finnish that over 100 years you'd end up with different accents and different variations of the original Finnish. The language would branch and merge and branch in unpredictable ways as it gets incorporated into various cultures and you'd end up with a situation similar to English today.
I don't think Finnish (or any language in a state of diglossia) is the best example here. That is, Finnish is already essentially two separate languages. Written Finnish (kirjakieli) is a somewhat artificial language that was a compromise between different dialects, because at the birth of Finnish literacy there was no longer an "original Finnish" on which to base writing.
Written Finnish is mainly a written thing, and while there is a close correspondence between the orthography and how it would be read aloud, when Finns actually speak they use spoken Finnish (puhekieli), which isn't standardized and varies from region to region.
I am referring strictly to the correspondence between a written language and its pronunciation. The fact that people speak differently from how they read/write is an altogether separate matter.
When reading Finnish, one pronounces the words very closely to how the word is spelled, regardless of whether when they speak they do so in an altogether different manner.
