The casting of guest roles was rather flawed on Five-O, to say the least!
Among the problems:
1) Actor/actress is too young for the role they're playing. Martin Sheen in "Cry, Lie" was a good example.
2) Actor/actress is too old for the role they're playing. Beverlee McKinsey in "The Joker's Wild, Man, Wild" was a good example.
3) Actor/actress plays an Asian when they're actually caucasian. Mark Lenard in "To Hell With Babe Ruth" is a good example.
4) Actor/actress plays a Native Hawaiian when they're actually caucasian. Paul L. Smith in "A Lion in the Streets" is a good example.
5) The acting is bad. This occurred many times throughout the series. It was forgivable when they used inexperienced locals for some small roles. It was less forgivable for some of the major guest roles, such as the two main guest actors in "Killer Bee".
6) The child's skin color doesn't make sense given the skin color of the biological parents. One that comes to mind is Paniolo, where the child of a light-skinned Hawaiian woman and an unseen "haole", has very dark skin.
7) The actress playing the role of someone plain or unattractive is actually very pretty. A good example was Loretta Swit, who played awkward telephone operator Wanda in "Three Dead Cows at Makapuu". (Incidentally, the character itself was very realistic, and the entire telephone operator center was authentically done. Swit was just too pretty for the role.)