Unlike other trading card games, you don’t need to manage currency in 40K CCG. You don’t need mana, Force, or bitcoins to recruit your fighting forces, nor do you need cards in your deck dedicated to generating those resources. As a result, every card in your deck can and must play a functional role on the battlefield. I believe this is a big point in the game’s favour.
But how do you know if a unit is worth playing in your deck? Let’s break down the characteristics of the unit card:

Flag: Obviously, flags are important to capturing sectors and winning the game. Knowing your deck’s flag count will give you better insight into your deployment decisions, such as when to deploy aggressively to take a sector, or resort to playing a defensive, stalling battle until you can deploy more flag units to capture the sector next round.
Firepower: Shooting is the most common way to delete opposing units, but its strength and prevalence amongst your units is dependent on each race. Some units barely shoot (Tyranids), others need a lot of synergy to shoot effectively (Orks), and some are just naturally great shooters (Space Marines, Chaos). Knowing how independent your shooting units can be, and how fast they are, will help you deploy an effective shooting force at a sector.
Speed: Speed is an attribute that applies both offensively and defensively. High-speed shooters will rarely be blocked, while high-speed blockers will consistently protect your crucial units. Note that units will rarely come with both high firepower and speed – you’ll commonly need to buff one or the other (or in the case of Tyranids, debuff) through tactics and abilities to optimize your shooting BAs.
Armor: As the main defensive attribute of a unit, armor can often hint at a unit’s role in combat. Many units are tough but slow, or fast but weak. Utility units often have low armor to offset their indirect but potent ability to impact the battlefield. As you learn each race, you will also begin to see how each has the ability to modify their stats such as armor. For example, expect a Space Marines player to commonly hold a +armor tactic in their command hand; conversely, a Tyranids player will have ready access to -armor tactics through command abilities or battlefield abilities.
Unit & Faction Types: Faction types will most commonly come into play as you play a subfaction such as Kult of Speed or Dark Angels. A unit’s types will be more common and relevant depending on each race, such as Line units for Orks and Imperial Guard, or Synapse Creatures for Tyranids.
Die Roll: As a concept familiar to Star Wars CCG players, the die roll of a card will play a part in how consistently your deck can pass its tests. In addition, it is a mechanical signal of how strong a card is expected to be. D1 cards are generally very powerful on the battlefield and sometimes in hand, whereas D6 cards are less impactful units but which make up their weaknesses though strong command lines and/or high die rolls. How you include both high and low die roll cards will be a balancing act to ensure your deck performs consistently.
Command Line: Your command hand can give you the edge you need to win a battle, or it can cripple your chances of taking a sector. While it’s obvious that you don’t want an E: -1 flag in your hand at the end of a battle, what’s less obvious is determining how helpful command lines can be to your deck. For example, a BA with a 6+ test is essentially a dud in your deck if you only have 3 D6 cards in your 60. Don’t underestimate the less-flashy but always relevant tactics such as armor or firepower boosters. Just as how you wouldn’t want a battlefield unit to be useless, you want to make sure your command lines are as helpful as possible to your deck. Sometimes, negative or suboptimal command lines are unavoidable – in which case, understand the risk you’re taking, and if possible, mitigate them through abilities such as discard.