That's just deflection of how easy Convoke on its own can be shut down by either an interrupt or any loss-of-control that you apply to the druid (assuming you don't out-range or LoS the spell). We can play the game of "well X spell combined with Y spell or Z class is OP" all we want, as that can be applied to tons of deadly spells or scenarios. If you let a druid get a full Convoke off, especially in a 3v3 scenario, you and/or your team did something very wrong. There are ways to setup a Convoke where it's very hard to avoid it, but that takes setup and coordination of a team... which, again, applies to any burst damage that isn't named Convoke.
Irony is that the more viable targets available, the less powerful Convoke really is. Using Convoke against a pet class, especially an UHDK, will more often than not have most (or all) your Convoke spells hitting pets instead of the players.
Even if we go back to the OP about being a warlock and using Netherward against a Moonkin yet the Feral getting a full cast into said warlock. If we're talking 1v1, there's a bunch of ways to deal with it from a Feral as a warlock. Mortal Coil, Howl of Terror, pet interrupt/stun, gateway, teleport, etc. If you have a partner, the options open up even more. If Convoke was a really short CD or could be recast if interrupted, I could certainly see a balancing issue. However, Convoke has a huge, glarring weakness that pretty much every class has a way to counter because it's a channel spell and has a 2min cooldown, along with the druid unable to control what is cast and being very vulnerable during the long channel. If anything, I'd say Convoke is probably more balanced than most deadly spells due to the ease of shutting it down in terms of its power compared to other spells/abilities that you'll encounter in arenas that have little to no recourse for most players beyond straight-up immunities.
So yeah, if you're getting destroyed by Convoke constantly, either the druid (and their team) are constantly outplaying you (and your team) or you're not playing against a Convoke druid properly. Out of all the potential deadly setups I've encounter in arenas this season, Convoke druid is by far one of the easier ones to shut down and/or avoid.