Most melee infantry has been various levels of useless in WH2.
Yes, but generally higher tier units are better than lower tier ones.
For VCoast, that was not the case. Depth Guard were terrible at dealing damage and even worse at holding the line. Syreens (a lower tier unit) beat them at both and the chaff beat them at line holding. There was zero reason to ever get Depth Guard.
At some point, a straight battle of one low-tier chaff unit vs one unit of Depth Guard saw the latter winning, but in a crumbling state dying shortly after. That's how absurd it was.
Now that they've been buffed a bit, they are... decent? You can use them now, anyway, to spice things up a little without feeling bad about it.
Also not sure what you mean by one viable strategy. They are a shooty faction, they shoot a lot. Is skaven a one viable strategy faction in SP because no one uses stormvermin?
I'm not talking about min-maxing.
Sure, every faction has one best strategy.
But Stormvermin are still viable, they beat their lower tier units at their job, you can win with them, they are "just" a worse option than others (or were, no idea about WH3).
But with VCoast you could put the shooty stuff behind crabs, because crabs are a) the best at holding the line and b) allow shooting through them as their models are far enough apart.
You
need spaces to shoot through because while VCoast is ranged heavy, all are gunners, meaning they get blocked by your own frontline and are too slow to move around to reposition (unlike similar gunners like the Empire ones).
And that's it.
There was just nothing else you could reliably win engagements with (ignoring doom stacks). Any high-tier army should be able to win a match-up of similar value. You could not do that with an army focusing on Depth Guard because of how absurdly fast they died.
Even Cylostra only makes the ethereal units cheaper. Not stronger. So while an ethereal themed army is cool theme-wise, it was just not good, not even with the ethereal legendary lord.