Nothing is above criticism. For art, not being criticized is the biggest insult, it means it is irrelevant, that it has no value to the world, that it might as well be ignored because there's nothing of interest to be said about it.
Art does not exist in a vacuum. It depicts a fantasy world, but it was written and drawn in the same real world that we live in, by real people who imprinted their lived experiences, their knowledge, their point of view, their dreams, their culture, their belief systems and their values to paper. How societal forces can shape individual forms of expressions.
The criticism is: what those are and what are the implications of broadcasting that set of values to the rest of the world?
What impact those values have in shaping our societies if they were to be absorved by the audience? How much do they reinforce the ideas that are used to justify real world injustice or how much they can contribute to the conceptualization of a better future?
The idea that slavery was natural during 'medieval' times, is not just historically imprecise, it leaves the implication that the forms of exploitation of labor today are also natural, necessary and that there's no other way things could be. There were other forms of organizing society in the past, there will be others besides capitalism in the future. But media can help create the perception that the current status quo is all there was, is and will be, so why try to change it?
As for slavery... the shadowrun games makes things very clear when they call people who work for corporations 'wage slaves'. The reference is not to the distant past or the distant future, its talking about us. Through the absurdist lens of a distopic future, it makes it clearer that we are living in an absurdist distopic world now. A fantasy is not just a fantasy, most of the time.
This in no way reflects the work of scanlators, their choice in series or the enjoyment of some aspects of the manga. It also means people care enough about it to post long tirades like this one. You need expectations and investment to be disapointed, if people didn't care they would just ignore it. So thank you and keep the good work.
Even if sometimes the author seems like they deserve a slap in the face, you have no reason to take it personally.