The "quality" of its elders is one of the JW's dirty little secrets. They dont like to talk about it, and criticism is not only discouraged - it is portrayed as immature or unspiritual.
Privately, however, the "quality" factor is an issue in most of the congos I have been in (which was five but I was a dub for 30 years, very active in the circuit and familiar with many more congos and their BOE makeup). Stories are being posted here now, and I go on an on about the human pond scum that floats to the top in some congos.
To your question: Elders are supposed to be recommended based upon the "scriptural qualifications" found in 1 Timothy, and these are normally reviewed line by line. In practice, it comes down to biased men expressing their opinion on whether so-and-so is "qualified to teach," for example, or whatever other point is being discussed. What looks like "qualified to teach" to me may look like a really poor choice to you, and vice versa.
Often the body is split on the decision, so for harmony's sake a compromise is hammered out. The group starts out to find a guy who meets "all" of the qualifications listed, and often settles for "most." If the guy under consideration is well-liked and connected to the right in-crowd, "most" is good enough... and "most" is also flexible. If he's not well liked, he's gotta have "all" or there's no way he'll be recommended. At least not "this time," which is an "out" often exercised by elder bodies. As in, "Well, he's probably not ready this time, but let's keep an eye on him for six more months and see." The CO visit is every six months, so this means he won't be recommended now but "may" be recommended in six months. Or a year. Or never.
Many "good" potential elders have been frustrated by this process. Others smell the coffee and attached themselves to the in-crowd to improve their chances on the next go-around, often after being counseled/invited to do by a self-appointed "sponsor" (unofficial title) on the body.
So, yes, there is a lot of behind the scenes maneuvering that goes into the process. Nepotism plays a very big role, particularly in small congos (it's much easier to waive or soften up the "qualifications" if the congo doesn't have many elders and "needs" more). Friendships are key. And fitting in is highly critical; some congos are "run" by two or three charismatic elders who wield all the power, and they often set standards for eldership that are above and beyond, or beside, the so-called scriptural qualifications. If you don't measure up to what they want in a co-elder, you won't be appointed. Many elder-wanna-be's change congos because of it.