I think Overwatch is a bad example because you can't turn the loot into real money. Valve's games are a better example because you can sell the loot directly through Steam -- in some cases for hundreds of dollars.
Overwatch is also a bad (good) example because the developers have a policy where only cosmetic items are available through the loot boxes, while all gameplay features are free for everyone.
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.
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")
Doesn't feel manipulative to me, just annoying. I wish there was a big 'open all boxes and skip animations' button that I could click every few months. Right now my boxes tend to accumulate because I just can't be bothered to watch the stupid animation, even when I'm waiting for competitive matchmaking, which can literally take minutes.
If this sounds kind of familiar, it's because a similar concept exists in many Arcades already, in the form of capsule machines. Japanese Gashapon are the oft-cited inspiration, but countless variations on this idea exist. Here in South Texas, they're a common sight in grocery stores and restaurants. https://en.wikipedia.org/wiki/Gashapon
On their own, random rewards in a video game sound tame, but where this gets questionably ethical are games which (a) accept real money for in-game purchases, and (b) offer loot-boxes as an in-game purchase. This enables game players to pay real money to receive a random reward, and it's not hard to draw comparisons to real life slot machines. Unlike real-life gambling though, loot boxes have been popping up in games that are often targeted towards minors, making the issue even more complicated. I won't weigh in on the issue here, I just wanted to try to clarify what they are.