If it's 100% cosmetic stuff and the game gives you freebies every now and then like Overwatch, I'm totally okay with it. 

Something like Battlefront 2/ Gears of War 4 where it alters in game stats and skills? It's inexcusable. It literally gives people a sense of urgency that they have to buy crates to keep competitive because the grind is excessive and you're trying to keep up with people who have way better stats and equipment than you. In Battlefront, for instance, Heroes vs. Villains is almost unplayable at low levels because cards make a significant difference. When some asshat with a lightsaber can take 40% more damage and has faster cooldowns on all of their abilities, you're often dying way more than you're killing because sometimes it comes down to a matter of hits. Starfighters are similar, especially when you are up against people with much better turning than you and have faster/ more damaging shots. When it's about dog fighting and trying to get behind an enemy, sometimes you only have seconds to inflict damage and if you can't turn as sharply as another player, they're going to get behind you and stay there.

Fortnite is actually okay for me for lootboxes, they all contain stuff that's pretty essential for progression, but the rate you earn them for free is pretty brisk and in gameplay terms, it's all PVE so you're not up against other players (I don't believe the Battle Royale mode has loot systems since it relies on you picking stuff up on the map, so it isn't relevant to my point), and since the game is coop, people with better gear tend to help you out a lot. It still comes down to your skill as a player, and I've still found my gear holding its own despite being several levels lower than the recommended in several instances.