Invoking spirits of the dead- for help, for divination, for assistance, etc- is common practice in many of the world's most popular religions. See, for instance, the cult of the saints in Christianity, or China's Book of Changes which is held to draw on the spirits of the ancient sages for guidance. The power of dead ancestors is important in religions as varied as Voodoo and Daoism. I would say religious cultures with an aversion to necromancy are a distinct minority, actually. Again it helps to have some knowledge of comparative religion- and an idea of what kind of culture prevails in the story- before making pronouncements about what is or isn't ethical. You and I may agree that blood sacrifice is unethical but people who believe that it is necessary to nourish the sun and rain and keep the world from collapsing into chaos would disagree.
Folks, most or all of us who grew up in the West were raised to think that the post-Enlightenment, post- Reformation values now in vogue are somehow timeless and universal. They are not. The sooner we all grasp this, the easier it will be to understand- or imagine- cultures with different values.
While not exactly 'taboo' I think a writer should avoid relying on magic that makes everything too easy for the magician. Including leaving no consequences. Magic CAN become a modern-day equivalent of deus-ex-machina, in the hands of a lazy writer. Oh there's a problem? Never mind. The hero or mentor has magic. Magic is here to save the day. Next problem.... If in the hands of the antagonist, limitless magic can hamstring the story possibilities as well. How do you stop somebody who is unstoppable? By definition, you can't.
It's impossible to answer without establishing the ethics, morals and religious code of the setting. Taboo, even in modern times, varies enormously. Polyamory, for example, or public nudity, varies on the taboo scale. What's considered taboo can also be influenced by social norms, with normal actions in one country completely taboo in others, like public spitting or men touching women they are not related to. magic would be the same. Why is it taboo? What is at play in society to make that kind of magic not acceptable? And, most importantly, according to who?
Unless heroes also have limitless magic... except that causes its own problems. Which is why I am personally a fan of "hard" magic systems. "Soft" magic systems can be useful, but only if magic is decidedly in the background. But if it does play a large role, then you need to have some hard rules - hard limits, at least - because otherwise everything can be solved with a snap of fingers.
I entirely disagree. A soft magic system allows you to do whatever you want, and have no need to fit it into a particular set of rules. Why does it work? It just does. Why can't they do this with it? Well, they just can't. Readers won't generally care. Hard magic systems are useful when the system itself is part of the plot. Take the Force. It apparently allows you to strangle people to death at a distance, fire proton torpedoes through tiny holes, lift spaceships and see the future in conveniently vague ways so as not to give away the plot. Why it works and what its limits are is never explained in the original films, and really needs no explanation to enjoy them.
I think, logically speaking, the ethics of necromancy depend on whether you are controlling spirits or more communing and contracting with them. Positive necromancy is usually portrayed more as the latter, while a lot of evil Western necromancy, as well as generally evoking the distaste for death and corpses, is often portrayed as exploitative.
