Another review talked about Hawkeye in a vacuum, as if it's a problem that he can only fire off all his arrows when allies thwart. And that's a surprising take to me.
See, even if Hawkeye is literally your only ally, he's still mostly a strictly better Tac Team. (Not quite because he can only target minions. But minions are the main thing you want to use Tac Team for through the game anyway.) He's Tac Team that can also thwart 3 as he goes. Or has the option of only thwarting 2, hitting for 4, and blocking an attack from the villain in a pinch. And more importantly, he's Tac Team in an aspect that doesn't have a lot of good options for minion killings.
While Tac Team isn't that good of a card, the biggest reason it's been abandoned is that there are just so many better options in Aggression. But Justice is an aspect that isn't usually the best at taking out minions.
But finally, this Hawkeye's real power is that he triggers off of any allies thwarting and doesn't have a limit per phase. If you already have other allies in play before you drop Hawkeye, you can deal 6 damage on the same turn you play him. Or you can play him and some other allies... and then just do nothing with them. Just save them for the moment a minion drops, and instantly kill it.
And you do only need one other ally out to make this work.
Most of the time, this is going to be "deal 4 damage and thwart 1 this turn, deal 4 damage and thwart 1 another turn, block one attack."
And that's pretty incredible value.
I wouldn't run this solo if there weren't a lot of other allies to take advantage of it in the deck. But I don't think it's fair to call that "niche" either if the so-called niche he's good in is literally any multiplayer deck or a solo deck with a lot of allies. There are few cards anymore that are such staples that they fit into literally every deck, especially with so few ally slots. And this is no exception. But there are a lot of Justice decks that do want him because there are a lot of decks that run lots of allies.
Where I found him most useful for me though was a Hulkling Avengers-tribal deck built around getting free readies with Earth's Mightiest Heroes. Because then, if there are no minions out, you can just use him to ready your character without feeling like you're going to waste his arrows.