Who was a disaster? Not funny, no acting talent, no ability to improvise, etc?

Cloris Leachman

Danielle Spencer (Dee from What's Happening).
She must have skipped acting school.

Courtney Cox

Cybill Shepherd! She was great in "Moonlighting", but she's not exactly someone you should build a sitcom around. Completely unfunny.

Courteney Cox is a great mention, though. Before "Friends", she was really horrible in "Family Ties".