Good grief! The Modernized version of Unmask is one of the few updated cards that just seems straight up more powerful than its original counterpart!
Unmask is typically played in Legacy in combo decks that don't really care about card advantage and just want to force their combo through. Modern is theoretically at a much lower power level, but we've all seen how easily the format can devolve into overpowered combo decks running into each other. Grief can provide free disruption to either protect your own combo or slow down your opponent's. It is worth pointing out that one of the uses of Unmask was in Reanimator decks to target yourself and put a fatty in the graveyard in a pinch, which Grief cannot do (though you could just reanimate Grief itself if you wanted). However, the positives most likely far outweigh this one downside.
Using an evoke creature to evoke the feeling of Unmask was a very clever design choice. But it also potentially gives us new ways to take advantage of the card type, since enters the battlefields effects on creatures are much easier to abuse and repeat than sorceries. In the games where tempo takes a back seat, we get a free 3/2 menace body for our efforts of casting Grief for 4 mana. An example of a small interaction that could be common in Modern is simply the ability to return Grief with Kolaghan's Command. If we wanted to go deeper on the Evoke mechanic, we could try playing cards like Ephemerate which actually seems amazing on turn 1. We can wait until the Thoughtseize effect resolves before choosing to cast Ephemerate, so we avoid the risk of being blown out.
Another avenue to look for including Grief is the Vengevine type decks of the world. Vengevine decks don't usually have a huge issue finding a second creature to cast, but getting a premium disruption spell that counts as a creature cast for free certainly can't hurt!