Machine-assisted translation is deceptively bad, especially for fiction content like manga. The machine TL might mix up the order of a sentence like 'bob verbed alice', fundamentally changing the meaning, alter context, or omit implied information. All of these errors will produce english sentences that appear sensical and valid and will get past most editors and quality checkers unless they're familiar with the source language or
very familiar with the work. This sort of error even creeps into official human translations of anime/manga in some cases when the translators are working quickly without a good editor (i.e. simulcasts where they have tight deadlines and low budgets)
In practice if you look at the fan translation community for manga machine-assisted translation is not given much more respect than machine translation - they both produce bad results and in many cases the people who normally welcome even a clumsy translation will reject machine TL and attempt to have it removed, because it often causes people to fundamentally misunderstand the work. The worst cases of machine-assisted TL in manga become infamous to the point of becoming shared memes - try googling "abaj" or "duwang" sometime.
For a very simple pervasive example: Japanese frequently uses gender-neutral pronouns and when translating to English you'll need to appropriately select the right gendered pronoun (or proper name) for each one, if you can. This is something a human can do pretty accurately if they have enough context and knowledge of the material, but it is nearly impossible for a computer to do it accurately without a ton of assistance. In a novel this would be an easier problem because all the necessary context is in the text instead of in the art and panel layouts. You'll note this arxiv paper intentionally cheats on the gender problem.