Star cards in Battlefront 2 alter your in-game abilities. For instance, you can equip an ability that increases the amount of damage you can withstand, or one that reduces the cooldown times of your abilities. The quality of these Star Cards is dependant upon their rarity — they're divided into common, uncommon, rare, epic and legendary — while unlocking duplicate Star Cards will improve the quality of the original card. Crafting parts obtained in loot crates can also be used to upgrade existing Star Cards, making them more powerful.
Though loot crates cost 1,100 in-game credits, making them relatively easy to earn by playing the game, it's inevitable that splashing out real-world cash on them will allow players to obtain them quickly while putting in zero effort. When such systems only rewards players with customization options this isn't so much of an issue, but when the abilities unlocked actually impact the way the game is played? Yeah, that's more concerning.
Edit: We should keep in mind that people were overreacting to Destiny 2's system as well, and that just turned out to be a whole misunderstanding. I think to get a better idea of this system would be to wait until launch to see how long it takes to earn the crates, the different ways of obtaining them, and the loot list vs straight up paying for them.
But does this make Star Wars Battlefront 2 pay-to-win? Unfortunately, it seems likely that this could well be the case, as players who pay out for loot crates will therefore have easier access to Star Cards, equipping a variety of abilities that will make them both more powerful and more resilient. Though it remains to be seen just how much Star Cards can change the tide of a match, if you're confronted by an opponent who has decked themselves out in legendary gear, your chances of besting them in a firefight are significantly reduced. Given that such gear is more easily obtainable by spending real money, it's difficult to argue against Star Wars Battlefront 2 being pay-to-win, with Star Cards designed to give players a tactical advantage.
While Star Wars Battlefront 2 has yet to be released, unless the ability to purchase loot crates with cash is removed from the game, it's difficult to see how EA could change the current system without it being pay-to-win. At this point Battlefront 2's loot crate system is designed to influence its gameplay, and those who don't spend money on loot crates will inevitably be left behind when it comes to unlocking high-tier Star Cards. It's always disappointing to see such a system implemented into a retail release, especially one that hinges upon the success of its multiplayer mode. Hopefully its loot crates aren't as pernicious in the full game as they appear to be in the beta.