Entomb and the fixed version of it, Unmarked Grave, are far more flexible, allowing you to set up spells with flashback like Past in Flames, additional static benefits through cards like Riftstone Portal, or simply open up more targets for you to recover through an Eternal Witness
Aside from a few cards here and there, the main difference between competitive Pioneer and competitive Explorer is that you cant play the Lotus Field combo deck in Explorer
You also gain 2 life by the by as if that were needed, and that's a very powerful free adjunct
A few years ago, there was a brief and glorious moment when Mastery of the Unseen was all the rage, leading to some absurd board states like this
Targeting the opponent can be upside in some spots, but a liability in others, and on the whole, Id call it a wash