However note that even something like "uppercase this string"
cannot be done the right way, because a truly Unicode aware
operation needs to look at the entire string (sentence), because
there may be interdependencies that modify the result.
If you are talking about downcasing Greek Σ, then it's true that always downcasing Σ to σ is inadequate. Unicode specifies that if the Σ appears before a space or punctuation mark, it downcases to ς instead. But this is not always correct.
For example, if the string "ΦΙΛΟΣ." is the word "φιλοσ" (meaning 'beloved' or 'friend') at the end of a sentence, "φιλοσ." is the correct downcasing. But if it is the abbreviation for "φιλοσοφία", meaning "philosophy", then the correct downcasing is "φιλοσ." So getting this right is an AI-complete problem which neither Unicode nor ICU can solve.