It still takes too much time to deal with him, unless it is late games, which usually doesn't happen if the Nando team has any competence. By the time you have dealt enough damage to melt his shield and his hp, rest of your team is dead, or almost dead. That is why he is so close to over powered. Time taken to deal with him is just too damn high.

Besides, it still plays on the Nando team's advantage since rest of your team is fighting 3v4. The resources tied on Nando especially on early game is too much unless you both have Nando (not happening in competitive), not to mention game has Jenos and Seris, who both can keep his hp up early game, due the lack of efficient cauterize and wrecker.

And yes, you can argue that kill the support first. Which brings us to another issue, Jenos is mobile and Seris is just slippery in general not to mention Pip or Damba and when you have dealt with the support/s, you have again lost time, then deal with Nando, you have lost even more time, leaving your own team under heavy fire and enemy probably has capped by now. If not the supports or Nando is back, when you finally wipe the enemy team and the whole show starts all over again. And that is the ideal situation, since you know, enemy still has 3v4 while you deal with Nando.

So yeah, Flanknando is quite a bit to deal with and either way you are screwed or you are playing against incompetent team.