this was never the case. anyone who used rainbow items after being max'd out on crit made a bad choice. not even sure why one would think that rainbow items were even remotely close to being useful before PVP system was introduced as literally only your main stat had any relevance. with the new pvp system one might argue that it has changed, but rainbow items got nerfed so I guess we will have to wait until they finalized all of the pvp upgrades
also stacking against your week sounds aweful. it might be the best to have single-main-statted items and duo items of your mainstat and the stat you are going to fight again so you can maintain high stats and still have good defense and not undergo the huge stat loss from rainbow items as crit/hp/and the leftover stat are prettymuch useless