Accuracy is a difficult subject.
The dictionary definition of accuracy is, "The quality or state of being correct or precise."
These two terms - "correctness" and "precision" - are very different.
For example, if my daughter asks me what time it is, and I say "lunchtime, noon, 12 o'clock, or 12:05" - all of these answers can be viewed as "correct." But they have varied levels of precision.
When it comes to numbers related to unreached peoples, the same thing applies.
One should not put too much stock in specific numbers attached to unreached peoples - like population numbers, for instance - as these are changing all the time. Globally, there is one birth every 7 seconds, one death every 13 seconds, and so while you're reading this, the population of unreached people has already change. Check out the population clock.
The main idea of statistical presentations about the unreached is to document broad "orders of magnitude." We are less concerned with it being 12:05 p.m., and more concerned with whether it is lunchtime, supper time, or midnight. From the point of view of a strategist or a planner or a fundraiser, the key question is not so much the digits in the number, but how many digits there are - "1,000" is vastly different from "10,000" or "100,000," but not vastly different from "1,200."
The number of unreached groups is driven entirely by two definitions: first, how you define "group"; second, how you define "unreached."
In terms of "groups," the World Christian Database defines it largely by language, and has a far smaller number than Joshua Project, which includes castes in India. These are just methodology differences: but when one researcher says there are, say, 4,000 unreached groups, and another says there are 8,000, it isn't that one is wrong. It's just that they are counting differently. (Same principle applies to counting missionaries: do you count short-term, long-term, tentmakers, BAM workers, etc?)
In terms of "unreached," the count is driven by an assessment of the number of Christians within the group, and their share of the overall population (% Christian). The numbers are only as good as that assessment. Researchers work hard to be 'correct' (truthful, well-researched, done with excellence). I have found, generally, that there are a small number of groups whose status as unreached is 'borderline.' Many argue over whether Catholic groups are unreached, for example; and some might question whether the Han Chinese are unreached. But the largest number of unreached groups are strongly non-Christian. No one would argue about whether the Pashot, or the Punjabi, or the Saudis, or the Uighurs were reached or not.
Is the research sometimes wrong? In certain instances, yes, the data is found to be incorrect and must be corrected. And data changes, too--peoples move, some are engaged, some are disengaged, and the data must be modified. We often discover new groups - what we do with the Deaf is an example currently being discussed. Yet a number of different approaches still results in the world's current status, so we know we have the "order of magnitude" right.
So: that the world is about a third to a quarter unreached can be generally understood to be correct but imprecise. It's one reason I don't get too hung up over the number of unreached groups; I focus more on the number of people, as a round amount ("a third to a quarter").