Clone Sergeant is extremely viable in the right composition, and anybody who says otherwise has likely not tried one out when maxed. Which is understandable. He's tricky to get and many would assume that anonymous toons that you get early in a game are more for training purposes and not really meant to be used late.
That's not an unreasonable assumption, but it's always better to listen to those who went ahead and actually tried it out at max. Having run a maxed Clone team for weeks in arena, I'll say that Clone Sergeant SHINES with Rex and Fives, but won't likely be very useful in other team compositions.