Actually, i don't think FT.Island and CNBlue are underrated. It's like calling TVXQ, H.O.T, SuJu, SHINee, 2PM and BigBang underrated.They are just from another k-gen. They were really popular back then. Moreover, they are bands, and Korea don't really appreciate this format (idk why. Just look at DAY6, N.Flying, The Rose and ONEWE. These bands are underrated). So, for K-Band FT.Island and CNBlue did really great.
As for fav underrated groups... For me it's SF9, A.C.E., ONEWE and DAY6