Black Widow & Punisher is an anime film within the Marvel Universe. It has two Marvel Heroes (the latter being more of an anti-hero) in an interesting pair up with a fairly decent premise. But the main problem this film has is its execution.
The art style and cinematography is great for the most part and does give a slightly different feel compared to other animated superhero films. It adds to the slightly mature tone and mildly gritty atmosphere. It also makes for some fairly good fight scenes, as they are well animated and punchy.
But the story and characters is this film's biggest flaw. The premise is decent, but the actual storytelling isn't all that compelling. Punisher's characterization starts off strong but ends up a little mischaracterised later on. His arc is interesting as he deals with having to hold back his need to go off the rails and kill as an anti-hero. But Black Widow's arc and characterization is just... meh. She's great in action scenes, but in the actual narrative she doesn't bring much to the table in terms of something interesting, other than a shoehorned love plot that has no compelling or impactful factors.
Overall, mediocre movie. It's visually good, but narratively lost.