No, is not so, shields don't take at all the full damage as if resistance/defense weren't there, try to play with crimson fist and you'll see that is not so in any feature of this game. What happens in arena is simply that the quantity of defense/resistance a ninja owns is very low, so the buffs are somehow advantaged because there are way less defense/resistance buffs easy available at the moment. But the reason why is so now is that since damage gets reduced by defense/resistance/damage reduction healings are not, so the whole system is balanced to take that in account.

Actually arena, as i said, is a special case due to the low values of defense/resistance the ninjas own there, so what is that actually matter in arena? Are debuffs, healings, cleanings and initiative.

While what you say is generally true it could also be counterproductive with many of the ninjas now we own and blitz teams are a double edged sword.

Lets take mb common blitz team. His damage is fully focused on their main, so, if you immobile or blind or it in round 1 before he uses lightning armor is almost surely your win (for example with *y harem jutsu) and if you are eventually able to let them use lightning armor and then chaos them is game over for them (for example you can do this easily with masked man + azure fang).

Consider that now also there's a very tough hard counter to any blitz team in arena that is the infamous han, roshi, sailor sakura, af team or any masked man team.