Well except when it doesn't, I guess. All combination of consonants may unpredictably get rounded down to f (wszystko) ą pronunciation at the end of the word has a wide range of sounds depending on regional accents, ę is the same but worse because it's usually ignored even in common words (część), you have all the usual Ci Cy Ca rules for palatal consonant, except these characters can also be part of longer sequences that are hard coded to a specific sound like szcz that is pronounced like "an untuned FM radio sound"
I would not be so optimistic to say 99%, but I think a lot of things like that could be actually assigned to some rules that are, well, actually applied pretty consistently. E.g. isn't devoicing of 'w' in 'wszystko' just the case of clusters of voiced and voiceless consonants? Similary 'Hodów' shows devoicing consonants at the end of a word.
I'm not sure about 'ą' - some examples would handy, but if we are talking about differences due to regional accents then following rules would be perfectly fine. With 'ę' - how do you pronounce 'część' actually? Again, I think the worst that can happen normally would be to be judged as 'ą ę'* ;)
I think that in general Polish pronunciation is fairly 'regular' and with applying just a few rules you would be almost always OK. Obviously I haven't try to learn Polish as my second language.
* For non-Polish speakers - if someone is 'ą ę' it means that (among others) he/she tries to be overly 'correct' in pronunciation.
Well ę in część is without the n never heard it with n, but I definitely use the n in words like jęzik, so yeah you cannot generalize between those nasals and "say n if it's in the middle of a word" only goes so far
Ą is a bit more regular at that, so usually loses the n when at the end of a word and never heard without n when in the middle of a word, except in places like Warsaw, and other cities up north, where I was called out as gòralski (which is funny considering I'm Italian, I guess I'm learning polish wrong but convincingly enough)
There are rules to devoicings, it's just that nobody bothers to remember them because you know what sounds good and what doesn't "by ear".
Sz/cz etc are just your regular dyphtongs, like English sh/ch/th etc - they are perfectly regular and phonetic. The only tricky one is rz, but it's also regular - there's a rule saying when you read it as ż and when you read it as sz depending on the preceding letter. And there's like 3 exceptions when you read it as separate letters - in words like "tarzan" :)
As for regional differences - all versions are correct usually. People in Kraków even accent on the first syllable and read "trzy" as "czy" :)
