With the amount of mobility and constant cc/peel getting stuff done as someone like 
is downright impossible. Even melees with only 1 gapcloser like 
struggle once they used their gapcloser and the enemy escapes. To make thinks worse, these champions aren't good splitpushers so they can't rely on that strategy.
xReverie1 votesThey're great in 1v1 situations, and boots of swiftness and a cloud drake can punish the enemy out of position. Work around your flash. Otherwise that's the disadvantage of the juggernauts - too OP if they had more mobility and stickiness.
doodlebob6661 voteshere is a list of immobile or 1 mobility spell melees who team fight very well
(ult kindof counts but its ussually used to get in a teamfight not for mobility during)
(undertuned but good in teamfights when not garbage) 
(same as shen)
(same as nautilus)
(same as shen and panth) (extremely strong in teamfights no idea why you say he struggles he just gets bullied out of jungle early) 
there are no juggernauts in this list because they as a class are based around situational power the same goes for a lot of other champs, if a team has a single super tank trundle is a monster since he can shut them out completely, if a team is dive heavy illaoi turns them into mulch 
is a monster fighting in the narrow confines of the jungle, 
given sufficent peel is actaully relatively strong in team fights, not worth an adc late game but still powerful,
