There are 6 finals that have /-u-/:
Cantonese /-u-/ is usually identical to Putonghua [u], which is like the English ‘u’ in ‘tofu’. However, Cantonese /-ung/ is actually Putonghua [ong], which is the ‘ung’ of the German surname Jung. As to Cantonese /-uk/, it is like the English ‘-ook’ in ‘cook’.
Since Putonghua [ong] is different from Cantonese /-ong/ but similar to Cantonese /-ung/, beginners are often confused. For more, see ‘Mastery of Cantonese Articulation – Mastery of the /-o-/ Vowel’.