tendency to pronounce acronyms as words rather than spelling out the letters?
I don't know the rules, but it seems that acronyms which look like normal words are prounounced as such.
Just like you and benszymanski noticed:
UFO, RAF (I think we don't use dots between the letters), other examples: ZUS, KRUS, PiS, MEN, NIK, FIFA, UEFA, UNICEF (closer to the English pronounciation ju-ni-cef or ju-ni-sef, not "unicef"), UNESCO (same here: ju-nes-ko, not "unesco")
read as a series of letters of alphabet (usually because they have no vowels, or have consonants clusters unnatural for Polish, so it's difficult to read them as normal "words" or they are too short, for example 2 letters only, even when one of those letters is a vowel):
PKP (pe-ka-pe), PKO (pe-ka-o, later the bank used this spelling and changed their name to Pekao), NBP (en-be-pe), PO (pe-o), SLD (es-el-de), LSD (I pronounce it el-es-di, like in English, but the PWN dictionary says the Polish spelling, el-es-de, is allowed too), SMS (es-em-es), MSZ (em-es-zet), ONZ (o-en-zet), MKOl. (mixed spelling: em-ka-ol), WHO (wu-ha-o).