zekrom is best early-game but begins to fall apart mid- to late-game because in order for it to do heavy damage, it also does 40 damage to itself, leaving it wide open for more pokemon to then OHKO a (basically by then) 90HP pokemon. you could tech in reuniclus to help zekrom or have yanmega in to be able to tag out zekrom and focus on yanmenga as main attacker after the first few turns, but both of those techs added in will slow your down in the early-game (which is where zekrom has the biggest advantage with pachirisu CL and Shaymin UL) but if done just right, those techs won't slow you down enough to where you'll still be competitive.
initially, zekrom was also used as a tech for decks that are weak to water, but after these first couple of months, players have figured out ways to battle zekrom's outrage without fueling it too much before the big KO, and building up zekrom's bolt strike in a non-lightning deck becomes very slow and troublesome, leaving it wide open for other decks to take their time to build up a big attack to take it out.