I've heard that "jugar" is the only 'u'-to-'ue' stem-changing verb. Is this true?
All stem changing verbs fall into one of three categories (e→ie(→i), e→i(→i), o→ue(→u), where -ir verbs do the second change). There are exactly nine exceptions in three special categories:
adquirir, perquirir, inquirir, preadquirir
cernir, concernir, decernir, discernir
The reason for jugar being a special case is that it was originally jogar (and thus o→ue) but for some reason as the vowel shifted the stem change remained.