I dropped this on volume 4 a week or two ago. In a sense it's worthy of praise because I can't stop thinking about it.
The premise sounds cool. You have a powerful weapon of war sent into the past, able to outclass everything else, with a full library of historical and war-related books as reference. But is this the birth of a new heroic 21st-century-minded mobile superpower? No. The crew is in need for food and fuel, and there is no easy way to obtain it.
For all their bluster, the crew is extremely foolish. They don't understand what kind of situation they're in. They have no reason to believe they will ever return to the present. Even if there was a chance, returning to the present would result in everything instantly not mattering anyway. And Japan has a major historical precedent for losing. So they SHOULD do whatever they can to make a place for themselves in the world, short of seriously fighting the Allies with their weaponry or turning their weaponry over to the Axis powers. But instead they only muster reactions once it is apparent to them that their hand has been forced, with no long-term plan whatsoever. I blame the captain of the ship for being a mellow old man who takes a laissez-faire approach to everything as if he's thinking "no precedent for time travel, so there's no right choice" and lets his crew waste their time on philosophical discussions intead of taking charge. Kosaka is a great character, but at least at this point in the story, he is not the protagonist. If he were the sole protagonist whose thought processes were explained well, I would perhaps be able to get behind him and this manga. I can tell from the ratings and comments that these are just my own personal pet peeves, but it is objectively true that the protagonists are failing to be strategic and suffering for it bar some kind of inconceivable advantage they gain later on to make up for it (short of undoing all their mistakes through more time magic, I don't see it happening). This is a good enough story that I'm engaged enough to be bothered by their mistakes, so I've dropped this to avoid tearing all my hair out in frustration with the behavior of the crew.
However, even if for some reason you're a weird enough person to have the same pet peeves as me, you've been warned by me just now, so you shouldn't hesitate to try this manga out. Obviously it has depth. I guess I'd just prefer a simpler, more feel-good time slip manga like Jin, compared to Zipang which has crisis after crisis being dealt with by characters I don't appreciate.