Not true. You just have to play the right thing.
There are some classes and specs that are very likely to always be...the right thing. Rogue for example. Rogues are always OP in at least one of their specs, and when they're not, it's only temporary before Blizzard buffs them. This is obviously for casual pvp like 1v1, wpvp and random bgs.
There are two types of PVP in WoW: Unrated and Rated.
Unrated: duels, wpvp, random bgs.
Rated: arena and rbgs.
Classes and specs like arms/fury warrior, shadow priest, elemental shaman, balance druid, warlock, are historically BAD at unrated PVP, while at the same time being good and very good for rated PVP.
Likewise there are classes and specs like rogue, ret paladin, ww monk, feral druid, hunter, that are are historically GOOD at unrated pvp, while at the same time not really shining in rated PVP.
And then there are the wild cards: classes and specs that seem to ride a rollercoaster between op-ness and absolute dogshit. I would put here enhancement shaman, dk and mage. You never know when and at what are these guys good, before they get nerfed or buffed and then nerfed or buffed again, as Blizzard doesn't have any idea what to do with these guys.
The main issue arises when someone playing a class/spec from one category wants to excel at the other category or at both, and there is no such thing, as Blizzard is incapable of balancing both pve and pvp at the same time. They can't even balance Overwatch which is a 100% pvp game, let alone a primarily pve game like WoW.