All ninjas are not standalone. You need to create an adequate formation to make them sustainable in battle and be efficient enough and of course, every lineup has a counter. That why you shouldn't just rely on one lineup and expect to win every battle. Even the rarest ninjas in the current meta are vulnerable if you don't put them in a suitable formation.

Unless you are trolling

Well in every version update we get new meta ninjas that can be fitted in new meta teams and each version features nerfs and buffs for some main character that outlines the best main character for each version lets say. So thats totally normal to see specific teams running in game most of the time in each version. At least the more updates we get the more diversified it can be and getting more possible meta lineups for each main character.

Still we are way behind end-game as we call it (always compared to Chinese version - which doesnt constitutes that it will be the same for us) but recently at least in past month we started getting several updates (Minato Hokage / Minato Jonin / Masked Man / Akatsuki meta Ninjas which still has other ninjas to come) so that creates many options for players to have more than one strong lineup that can fit best for each event (Matsuri / SWB / Arena or Bonds).

So from my opinion the battle game-play is not boring yet the daily events are progressing slowly that it became a routine where is might feel boring for some players.