Scientists recommend that humans consumption of meat is unsustainable to life

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The big problem with meat, health-wise, is that even in hunter-gatherer societies it's not something you eat every day (with the sole exception of Inuit cultures, who gorged on seals and fish everyday). Hunting is a very energy intensive activity that is often unsuccessful, and many times such societies had to rely on foragers for extended periods of time. Undoubtedly the increased reliance on meat-eating was good for human evolution: it meant less energy had to be expended on processing bulky and cellulose filled indigestible plant matter, allowing more resources to be diverted to the hungry brain; but the notion that primitive man ate meat as much as we did is wrong (and they of course had to expend effort to obtain their meats) - just as beneficial for the spread of humanity is the fact that we are ridiculously generalist in our eating habits. However, it's even worse in humans descended from agricultural societies (i.e. everybody on this forum, as far as I know): humans from these groups became adapted to starch based diets, with the expectation that meat was a relative rarity; the introduction of meat as a daily meal is an extremely recent change.

The other problem is processed meats, which often have their own problems: this includes as various compounds formed when smoking or cooking meats, as well as perhaps several common preservatives.

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Well fish has its own problems. An awful lot of ocean plastic comes from nets and other such junk, and both fish farming and using natural fish stocks create their own problems.