Black Widow & Punisher is set in the Marvel Anime universe, alongside several TV shows and Iron Man: Rise of the Technovore. That means the dialogue, action and characters all follow very Japanese animation tropes and styles. In many ways that's quite fun, but it does have some negatives.
Chief amongst those is the script, which feels stilted and poorly paced compared to more Western animation. Part of that is just the differences in intonation and phrase emphasis between the cultures, but I imagine a large part is that the translation has been done a little too quickly and forced to fit mouth movements. The result is passable dialogue that occasionally leaves you scratching your head, and which frequently veers wildly away from the standard characterisation of the main heroes on display.
Notably, the villains aren't as badly impacted by the disjointed script because they're basically brand new. Whilst the terrorist organisation Leviathan have a long history, they ultimately just exist here to provide nameless henchman for beat-downs. The main villain is Elihas Starr, whose name fans may recognise as that of the villain "Egghead", but beyond the name the two are utterly different. This version of Elihas is still a scientist, but one that was also a S.H.I.E.L.D operative, close friends with Fury and Black Widow's ex-lover, thought dead. Elihas is an interesting character, largely because his motivation is something other than world domination or money, but rather a desperation borne out of perceived inadequacy. He believes that, to truly have a shot at a life with Widow, he must become more than just a world-class scientist. Yes, it is a bit contrived and no, it doesn't have any hidden depths, but it just about works. The one down-side is that it doesn't really give the film much in the way of stakes, with the super-serum soldiers appearing pretty easy to take down, but that actually allows the script to focus more on the Punisher and his place in the Marvel-verse, which is much more interesting.
Script and slightly dubious villain/love-interest subplot aside, the rest of Black Widow & Punisher is a decent attempt at a fun superhero storyline. Focusing on two non-powered heroes allows for a much more stealth-based plotline, though action is still ever-present and wonderfully animated (although the amount of backflips Widow does is often, shall we say, overzealous). Animation in general is good, with clever frame transitions, good use of light and smooth movements. It is, as mentioned, incredibly manga-esque, but that's part of the fun. Action sequences are generally well choreographed, though the fist-to-fist punch stand-off trope is used far too regularly. The final showdown, for which the Avengers and a weird assortment of Z list villains show up to help with, is well-paced and interestingly put together. Plus, character design in general is just interesting; not great, but as anime variations go, these are done well. Overall, definitely a significant improvement over Rise of the Technovore.
The end result is a perfectly acceptable movie. It starts a little rocky, with some slightly awkward dialogue, but once the plot is well underway it's watchable enough. There's nothing particularly new or unique on offer here, but what you get is interesting, entertaining and fun.