Okay, so originally, I was going to put this under "mixed feelings" or even "not recommended," because the anime is flat out weird, and the multiple plots points it tries to tackle in the first few episodes destroy the pacing. Fortunately! I can say with quite proudness that I would recommend this to someone. Once you get over the hurdle which is the beginning, the show becomes really engaging. I mean, it's ninjas fighting ninjas, what's not to love?
This anime is definitely a fever dream. As someone who plays Kingdom Hearts, Under Ninja is likewise hard to follow along, and is something one has to be paying attention to if they want to understand the story. (Of course, one could also not, I think its enjoyable enough even if you didn't understand it at all. The gags are frequent and silly enough to keep a viewer engaged.)
I have a couple of bonus thoughts if you care to read them:
-So, the English voice acting in the sub is actually pretty good. Whoever was responsible for writing the dialogue made it sound janky, but I applaud the voice actors for barely having an accent.
-There are weird, perverted scenes in the anime? Not quite fanservice, but definitely ones that would make one uncomfortable if they didn't like seeing that stuff. (like stealing underwear)
-It's probably better to watch the anime (at least the first few eps) all at once. The beginning tries to start the series a month after Kuro takes on his ninja job, and it keeps flashing back to him starting the job to return to what he was doing in the present. (Its janky, but it does do a good job introducing all of the characters, I just wish they did it in some other fashion that wouldn't confuse the viewer.)