I would like to know what leaderboard system you used in your games and why. It would be great if you share some thoughts. Also it can help someone in future jams

In our project I used lootlocker leaderboard, so I implemented while label registration. I think registration scares people so it is main disadvantage in our game.

Lootlocker with automatic guest login.
On the first login I let them enter the desired (unique) username and then save the data locally for automatic login on every startup.
With the very little documentation and/or information online (unreal + lootlocker) it was not super easy but I feel the same way about the general fear of white label registration so I preferred to improvise.

We used LootLocker as well, just whatever the example code was, with the optional name change functions. Seems to work fine.

I used Leaderboard-Creator of Danqzq https://danqzq.itch.io/leaderboard-creator (which is free and uses TLS, I checked with Wireshark). At first, I tried LootLocker, but we wanted one leaderboard per level and I was afraid the limit displayed on my LootLocker account (1,000 users) would not be enough. During the tests, the number of users was rising too fast to not be scary.