This card is very narrowly applicable, mostly interesting for spirit Legolas when paired with leadership Gimli because he will plan to use both stats every turn. Even then it's not so exciting, 2 resources for 2 total stats.

Honestly the mirkwood knife is quite the disappointment imo.

At this stage of the game it has been clear that 1 cost for 2 stats is the usually the price to pay for great attachments Dwarrowdelf Axe, Gondorian Shield, Rivendell Blade are all for the most part 2 stats for 1 cost. Other good attachments are also 1 for 2 but conditional, like Bow of the Galadhrim, Dagger of Westernesse or Warrior Sword.

Silver Circlet is probably the exception to this rule, 2 stats for 2, but willpower is the best stat in the game and for a dedicated questing hero there's little else you want.

The mirkwood long knife has the same "efficiency" of 2 for 2 but has its stats divided and thus it severely limits it's potential. You'd have to run action advantage to even get 2 stats out of it, while action advantage on a Gondorian Shield for example can mean 4 points worth of defense.

It's also restricted and limited to silvan heroes, severely limiting again the applications for it to the point where there's virtually 0 places where another option wouldn't be better. Imo making it not restricted or allow it on allies would have open more space for this attachment to shine, but as it stands there little reason to play it.