I've only played Orks, so my perspective is skewed. I'm trying to wrap my brain around the economy system, specifically the Escalation phases. Early Escalation means you get less req/power per point, but a partial cost of units lost is refunded back when you lose them. As escalation goes up, less is refunded, but output out of captured points increases.
First thought coming to my mind is that early game you want to establish map control by preferring new units over resource upgrades, then going for the latter during Phase 2, so that as Phases 3 and 4 roll in, your cash flow gets a nice big boost.
In 1v1 I didn't feel like there was much of a gain towards investing in resources early on. You lose the ability to control the map, and could quite easily say goodbye to your investments as you're overwhelmed by superior numbers.
Trying to understand if the game will allow different types of openings. Eldar and SM might be more nuanced, since they have to upgrade their strongholds to unlock better tech, whereas the Orks use Waaagh! towers, and those have an impact in early skirmishes. Building one permanently unlocks one Waaagh! level, so even if you lose it, you still get the tech progression it allowed. It also acts as a light turret securing advanced positions, and there's always the cool and decisive Waaagh! buff.
In short: my impression so far is Orks play a very focused, aggressive early game, foregoing resource upgrades for units and Waaagh! towers, then capitalising on early aggression by switching to resource upgrades during Phase 2 to either keep the pressure, or catch-up on counters.
I do need to play more, of course, and experiment with investing on resources early and rushing towards a specific high-tech unit, and then see how much of a game changer that can be.
What were your impressions!? Can't wait to play more!