Blizzard invested a bunch of money and labour into creating various different sets of quadruplets of lore, quests, items, abilities and other content. They have quadrupled the work they had to undertake on class design and development beyond the inherited baseline abilities, except to the extent that they had to be changed to accommodate the covenant content.
I doubt that anyone could reasonably argue that the game does not have a host of problems, most importantly the constant underperformance of certain specs, with the degree and frequency of underperformance being proportional to the extent that the pertinent specs are not popular amongst the player base, even before a feedback loop.
I don’t disagree that a utopian state of complete balance and equilibrium will never be achieved, but the way the most recent batch of borrowed powers and adjacent content have been designed is quite evidently not the best way to utilise resources if one had any hope of achieving, at the very least, an attainable, realistic level of balance.
It is entirely possible to design a kit for a whole class or a particular spec which is aggregately and individually useful - where abilities are individually powerful and they work together well as a machinery. Given the extraordinary amount of resources they have spent on churning out an unnecessary volume of stuff at the expense of mechanical and design work, I would say that asymmetrical balancing is something which Blizzard had knowingly decided to forego.
It was all along a possibility to balance classes around their contributions other than personal dps, but instead of meaningful and asymmetrical personal contributions, we got Hogwarts houses with mostly jarring and clunky abilities. Truly, it is difficult to label the left behind specs and left behind covenant choices as anything other than a vainglorious vanity project and a balancing misadventure the solitary purpose of which is to lure players in with promises of the latest round of dull borrowed powers, because it is cheaper and makes for better advertising than stating “Our classes are functionally complete and well balanced.” Consumers prefer Hogwarts houses.
As to the state of Venthyr warriors… My understanding is that, at least theoretically, there should be parity between the covenants in terms of the size of their respective power budgets. Very simplistically, a spec which is not the best in terms of single target dps should still excel at something and it should surpass all the other specs in a specific, niche aspect which makes it desirable. The obvious one is higher single target dps, but that is not mandatory. Something else might be completely appropriate, such as movement, survivability or some other utility, but currently Venthyr just appears to have a much smaller power budget more than a case of adequate asymmetrical balancing.