Right, Overwatch is a relatively tame example where it starts to feel potentially manipulative. I forgot about the Battlefront scandal, that's a bit more obviously bad. Again here, the presence of random boosts isn't inherently a bad thing, but making them purchasable is already questionable ("pay to win") and putting them in real-money loot boxes is certainly sketchy ("pay to win but only sometimes")
Many other loot box games offer items that increase player power or change gameplay in loot boxes. For example, the prerelease version of Star Wars Battlefront 2 (2017) had a backlash because of items such as "Deal 30% more damage with ship blasters." Other games may have items such as "Deal 20% more damage, but reduce health by 30%" which open up additional strategic options.