U is an unusual vowel. A, e, i and o have names that echo their long vowel sound - ay, ee, igh, oa. U's name (pronounced yoo, two phonemes, y-oo) matches its sound only part of the time (unit, cute). On other occasions, u makes an oo sound (blue, rude). And all too often it doesn't even make an appearance in words with these sounds (new, shoe, zoo).
There can be problems also with short u. Southerners have two ways of pronouncing it - the shallow u of cup, sun and butter, and the deeper u of put, pull and push, which is the same as the vowel sound in look and could. Northerners have no such petty distinctions.
Working with other vowels, u is responsible for a wide assortment of sounds, au (aunty, sausage, taut, gauge), ua (dual, suave); eu (feud, Europe) ue (blue, cue, suet); iu (Julius) ui (fruit, ruin); ou (you, your, count, country, colour, rough, though, trough, could . . . oh, and goodness knows how many more) uo (duo); uu (vacuum, continuum).
It is also usually found lurking behind the letter q, and quite often behind g (guide, guard, language). It also turns up in build and buy because of their Germanic origins, and in biscuit, the Old French word for twice-cooked.