here is no mmorpg that offer you 100% successful upgrade, let's think about top mmorpg today which is black desert you can grind for months and when you upgrade accessory you can succes or fail when you fail your item and all the money is gone, so upgrade system in dbo is pretty easy compared to black desert, players already can go easy +11/12 is enough, before in Tw/hk when you had gear +8/+10 you was happy and gameplay was harder, today the standard is +11/+12 and gameplay is already easier, cc/grind etc... Daneos won't change it for sure he already make upgrade system easier 100% successful upgrade to +15 will ruin this game no one will upgrade item and wasting money before the final cap, until lvl 70 game economy will dying because everyone will collect money for get stones at final cap, that's mean all lvl caps before
the final will be useless.
But Blade and Soul offers you a 100% success rate, the hard part is farming materials.