I always had trepidations of flying vertical speed mode during climbs and FLCH mode during descends especially in IMC due to potential for accident in case of engine failure. During climb in vertical speed mode auto pilot will maintain given rate of climb despite luck of power wchich could lead to stall and during descend with FLCH AP will compensate lost power with pitching down in order to maintain assigned speed. Perhaps thats not the case with bigger jets.