Dawn of War III seems to only have Marines, Orks and Eldar. Let me know when they add the Tau.
Strange isn't it, putting three of the most popular armies in the game to start with.
Weirdo yanks and your weeaboo commie space cows. 
I thought Warhammer 40k was British.
Oh UP you are so thick aren't you?
The Tau were created to aim at weeaboo yanks. It worked. Doesn't change that they are commie space cows. (and i say that as someone who has always wanted to sort a Tau army some day, hell I've even got plenty of minis)
First, I fully understand why its Marines, Orks and Eldar. I'm just a stubborn old space communist.
Anyway, you're really selling the Tau short here. The Tau are my favorite faction in 40k and it has actually practically nothing to do with their anime influence (I say "practically" because I love the Tau aesthetic). In a word, the Tau are best described as "contrast."
Their methods are the least brutal of all of the factions. They actually bother to make and keep allies and always try to convince a conquest to join the Empire peacefully before bringing the hammer down. It rarely amounts to more than asking, getting a "no," and following by pointing a big gun at you and asking again, but this is still the least aggressive faction in the setting.
Their tactics are based on real life military tactics. They focus little on close combat and heavily on synergy and ranged target priority. The idea is that, yes, running up to a Tau and punching them is the best way to deal with them, but good luck getting there.
They are heavily focused on practicality. Aside from Farsight, you're never going to see a Tau charge headlong into battle, swinging a sword around. Ethereals carry honor blades, but they do so more for ritual and ceremony than combat. They wear armor that reflects this focus on practicality, with only one shoulder pad over the side that faces the enemy while shooting. They mount dangerous field testing equipment on drones to prevent unnecessary casualties.
They understand that technology is the means to an end, not magic. They make extensive use of AI and they're constantly trying to advance their tech. It can be argued that not only is their technology on a higher level than the Imperium, they may possibly be on a higher level than the Eldar. There is no mysticism in how they use and create technology. Every technological wonder they create is a tool, created for the purpose of advancing the Tau Empire.
They are the only faction that's growing instead of stagnating or regressing. The Imperium isn't afraid of what the Tau are, they're afraid of what the Tau will become. If just about any faction were to make a concentrated effort, they could crush the Tau, but people often forget that the Tau were discovered by the Imperium around M.35. When they had only just discovered fire. In 6000 years, the Tau went from cave dwelling primitives to a galactic power with a rapidly growing empire. Where was humanity 6000 years after discovering fire?
The Tau were designed to contrast the rest of the setting and that makes them far more interesting. It's easy to dismiss them as an attempt to court anime fans into 40k or try to say that they're out of place in the setting. The only problem there is that being out of place is the entire point.
In the tabletop, is Waveserpent spam still viable?
Whether it is or isn't anymore, I couldn't tell you. What I can tell you, however, is that no one cares about Wave Serpents because Wraithhost broke the game and now we wish the worst things we had to deal with were Necron flyers, Grey Knight Paladins and Wave Serpents.
