GNW tenten has basically 2 things going for her is that her mystery has no Battlefield cd and has a 10 chase.

Reason why her mystery is so strong is that it normally has a 80% buff on it with a 15% crit buff and its all elements which at base gives 20% more damage

Really Tenten not even a problem if we dissect the water meta you will see the main focus is on many things mabui - heal and charka, mei - barrier, immoblie and mystery, tenten - buff and mystery and water - mystery, heal and buff

You can easily see how pivotal mei is as the closer, mabui and tenten combo as the opener and water as the insurance. Tenten has no real strength alone (except Lightning Main like wtf op much) and basically all the new jinchuriki are one man armies so tenten teams are dying out