Hot take: Leona is actually terrible with Samira
Well, title. Leona is actually terrible support for Samira (in current meta). First of all, generally, enchanters are currently better than engage supports. Second, Leona, unlike other engage sups, is much more harder to follow up for Samira. Thresh pulls champions to him, Pyke pulls champs, Naut pulls champs, but not Leona. Which means, that there are many cases, where even your passive cant make it up for distance, that you need to go, making disconnect between engage and follow up. Third, Leona deals basically 0 damage in comparison to other sups. She is tanky, yes. But you need to make up for her lack of damage. Finally, more ranty and anecdotal, but majority of Leonas are terrible. They cant find engage window ever, they cant threat your enemy at all, they waste all spells