But if Thea's resurrection had issues later on, why would/will they use it on Sara?

As would I…

Darhk Red Arrow confirmed.
Perhaps Thea would choose not to use the pits, or Oliver may not want to see her full of darkness again.

Oliver is gonna say Laurel's name at his and Felicity's wedding.

Must be the Lazarus Pit. Still feeling the effects after a while.

When Tommy, Moira and Thea died, Oliver didn't resort to killing any of their killers (except for Ra's, but that wasn't for revenge). Tommy was his best friend, and Oliver worked together with Malcolm. Slade killed Moira, but Oliver didn't finish him off. The person in that grave must be someone so important to Oliver…

My predictions for this season:
-Darhk will kill Felicity, probably right after Oliver proposes to her (no-one has made that much of an impact on Oliver except for Tommy and his family)
-Lance will die (defies Darhk and gets the poison touch)
-Thea will turn 'evil' and join Darhk (Her dark side will take over because she…