WAAAGH in its current form is kind of bad.

Questionable

Not at all to say that its weak, just that its not a good mechanic, at least in its current form.

As the WAAAGH mechanic is currently, you get to double the size of all of your armies for 20 turns for doing basically nothing. You will always have a passive WAAAGH meter income and will only lose any waaagh points if you lose an army, in other words catastrophically fail at the game... Its incredibly strong, to the point where when the AI triggers a WAAAGH they become highly obnoxious unless you have lightning strike, and the fact that its inarguably powerful is part of why I think its not very good.

Really the only major issue I think there is with the WAAAGH mechanic is how WAAAGH is generated and what determines how many units you get in your armies. As it stands you can sit around doing nothing with a 20 stacks of the cheapest goblin fodder available to you and then suddenly have 20 free, much higher tier units attached to your pile of trash army.

Not only is it a bit silly, its also not very thematic. Orcs thrive on combat and war, they want to get out there and fight or they'll get bored, and while the initial implementation of Greenskins was quite flawed I think the Fightiness mechanic represented that intrinsic part of the orc psyche quite well.

My suggestion for improving the WAAAGH mechanic, both making it more thematic and more balanced is to bring back the Fightiness meter and tie the generation of the WAAAGH to that. Instead of directly generating WAAAGH by razing settlements and winning battles, have these things build up the Fightiness meter, buffing the morale of your boys and getting them real riled up for a proppa scrap. When the Fightiness meter is high, word gets around of a real green git out there gettin' some real good fights and that is what should generate the global WAAAGH meter. Conversely, having idle armies that aren't out and about raiding, looting and razing would have low fightiness and that would reduce the WAAAGH meter, because what propa git would follow some lazy bum who don't even got the grit to find a fight.

Finally when the global WAAAGH meter is filled and the WAAAGH is called to action what should determine the size & quality of your following WAAAGH army is not the mere number of units in your army but the size of your Fightiness meter. So even if you have a roughed up and haggard army if they've been getting into a lot of fights and winning battles, more boys would want to join that army rather than the 20 stack of goblins who've been hiding in a settlement for 5 turns.

In my opinion this set up would both be more fun to play with & against and be much more thematic and flavorful for the orcs as a whole.

TL;DR give Orcs back a victory meter like Bloodletting and have that contribute to the global WAAAGH meter rather than a constant passive gain, and have WAAAGH armies join armies based on how good their Fightiness is rather than the pure numerical value of the army.