I'm surprised there isn't a review for this.
It's good. It's really damn good. I've played Hank Samson once in a Standard Dunwich campaign and this card was often really useful, but I played it before I realised I couldn't use it against that one enemy we all hate, so I would probably play him again in a different campaign. Testless damage is something that gets much better in Hard / Expert, but is still great in Standard / Easy.
It's best to save Stouthearted for when you flip as it is functionally some of the only healing you have. (Note that you still have soak and whatnot. Cards like Wrong Place, Right Time and "Devil" work too, but Stouthearted does it the most reliably and efficient way in my opinion. Peter Sylvestre / Peter Sylvestre(2XP) + Jessica Hyde, Perseverance, shit like that.)
Since Stouthearted is fast, it lets you put out a burst of extra damage in a turn without sinking an action on it.
However, there are four minor caveats to Stouthearted. The first is the timing restriction, which is probably the most frustrating part of the card. You can't just play it any time and get value out of it, you either have to draw an enemy or engage an enemy that's engaged with another investigator to play it. Normally, engaging is a loss of tempo, but with Stouthearted you mitigate that loss by dealing 2 damage, which is generally an average amount for fighters. To combat this, what I usually do is run separate engage cards such as "Get behind me!", "Get over here!" / "Get over here!" (2XP), and Heroic Rescue / Heroic Rescue(2XP). Personally, I think "Get over here!" (2XP) is the best card to pair with this as it potentially lets you do upwards of 3-4 damage fast(Stouthearted + 1 damage from successful fight + any other potential damage boosts).
Second of all, you need to have damage/horror on you. This really isn't a problem as the mythos is more than happy to dish out damage/horror and you can always take damage/horror through beneficial card effects such as Blood Will Have Blood and Fend Off.
Thirdly, you cannot recurr it without looping for your deck. It's not a card and it doesn't have a level, so there's no way to get it back. And that's okay. Just using this once is beneficial.
And lastly, this doesn't work against Elite enemies. I'm not surprised. You'll still have plenty of applicable targets to use this.
For two resources, a card, and some awkward timing restrictions, you have a pretty damn good signature card that makes you feel like you're a powerhouse of a tank.