No, of course it's not unbiased.

Literally nothing that humans produce is free from bias -- we are each a product of our own upbringing, our own place in the world, our abilities or disabilities, our language skills (or lack thereof), our cultural context, who we studied with, etc.

That's one of the reasons it's important to read widely in any given area, to suss out where an author's biases are and how their works fit into the general historiographical tradition of an area. "Bias" is everywhere in what we do.

Now, the question you probably are actually wanting to ask is: is the booklist accurate, that is, does it fairly represent the scholarship around individual areas; and, as part of that, is it comprehensive of scholarship in the areas that are represented and the way that it's organized?

I would say that at least for areas I'm familiar with, it's a good fair introduction to the scholarship around a topic. Let's take the example of the booklist on early modern Britain; there's a fair amount on the English Reformation there, but we also get the classics from EP Thompson and Linda Colley that explore state formation from a couple of different perspectives. If you were to read the books on that list you would have a good broad overview of early modern Britain, though arguably one that's a bit skewed towards religion as a driving factor (though balanced out with some good old-time Marxism). I could probably expand on that with a few more titles and have been meaning to, but there's nothing outside of the mainstream there.

Speaking of adding to it: The way the booklist is generated is that our flaired users who are part of our Panel of Historians can add books to the list and edit it at any time; and the mod-team periodically runs drives to clean it up (mostly to make sure it's organized semi-sensibly and the formatting tracks) and add books to the Goodreads version of the list.

At any given moment, AskHistorians has somewhere between 300 and 400 flaired users who have demonstrated their expertise in a wide range of subject areas. Every one of them has the ability to edit the booklist.

The two most important results of this are (1) not all sections of the booklist are equally populated and descriptive (2) the books listed reflect the interests and reading experiences of flairs. Take a look at the list of flairs; you'll see how heavily European and American history figure. Guess what effect this has on the booklist.

There is no overarching authority of the booklist; it's set up as a wiki. Frankly, this works to maintain a quality booklist of scholarly (and reputable pop history) source because and only because of the high standards AskHistorians demands of answers in the subreddit. Anyone enthusiastic enough to earn flair is probably committed enough to our level of intellectual conversation that they're not going to try to slip in maliciously some Ancient Aliens book.

So tl;dr: is the books list unbiased? No, because nothing is. Is it generally fair to scholarship on a given area? Sure, as best as we can make it.

Edit to add:

in terms of facts vs opinions, is this sub's recommended book list in general unbiased or is that not a criteria for the list?

Something that I would add is that historians are not stenographers; the discipline is one of taking what we know about the past and building a narrative out of it that respects historical events but also synthesizes them, analyses them, and places them in context. It's perfectly reasonable for historians to disagree over the interpretations and importance of an event, and our understanding of how to study history changes over time -- not just with revolutions in the field, such as Marxism or the Annales school or postmodernism, but as new information is discovered and new ways to study what we know is made possible.

An example I like to bring up is the historiography around the battle of Midway: during WWII this was portrayed as a heroic American victory against an invincible Japanese armada, where three US carriers faced down the entire might of the Japanese battle fleet. Much of the scholarship about Midway continued in this way; Gordon Prange's Miracle at Midway is perhaps the pinnacle of this. But, it contains serious misconceptions about how the Japanese conducted their carrier operations (he assumed their practice mirrored US practice), it uses Japanese sources (in translation) very uncritically, and thus it has some serious errors. (It is worth mentioning, perhaps, that Prange did not speak Japanese and thus could not read Japanese primary sources.) Other historians, perhaps most notably Jonathan Parshall and Anthony Tully (but in collaboration with others such as David Evans and Mark Peattie) spent a great deal of time in Japanese archives to tell the story of Midway from the Japanese side of things, and now we have an account of the battle that overturns many of the myths from Prange's time.

Is that an example of bias? Maybe so, but it's the particular bias of writing from a specific time and place; the bigger issue with Prange's work is not ideological bias but a failure to engage critically with sources and an overreliance on veterans' memories (particularly charismatic veterans who spoke English, specifically Mitsuo Fuchida).