Rhoticity
en.wikipedia.org
In New Jersey, Pizza Paula is a pizza parlor,
And Bostonians drop the 'r' at the slightest whim.
Rs vary much with dialects in any language, mine is written upside down phonetically, for instance.
It's vowels that in my opinion are most problematic in English: basically all reasonable pronunciations should be accepted, this authoritative guy explicitly confirms it:
In all Romance languages there is a one-to-one correspondance between a written word and its (standard) pronunciation. Vowels can be pronounced differently depending on other sounds before or after them in the context of a sentence, but also these variations are standardised (except for French where some clusters of vowels can be messy). German is like this, Greek as well, they both went through lexical reforms and standardisation to improve their regularity. I bet also slavic and scandinavian languages follow this pattern. Probably also Gaelic.
Not English, where you basically have got to know the pronunciation of every word, as looking them up on the dictionary won't help with the spelling, unless the dictionary reports phonetic spelling and you happen to know it. The written word is stuck on how pronunciation as it was in the XVII century (Rs are rolling and Us are to be pronounced as French OUs).
To improve pronunciation for new learners, adoption of some sort of runic alphabet would make more sense for English, as things are now.
Maybe just write down consonant sounds and leave vowels to be represented as fillers, as in Arabic script.
Or just adopt As and Us to give a hint as to whether the vowel sound will be from the mouth or from the throat, because right now the other vowels in the middle are confusingly representing 2 dozens different sounds.