Contact with birth parents is not always a good idea

After listening to an interview with a woman who had been abandoned as a baby, and who had spent 30-odd years attempting (unsuccessfully) to find her birth parents, I couldn't help but think that, were I in her position, I would probably not even try.
Then I got to wondering how many people who do succeed in contacting their birth parents actually have happy outcomes. I kind of assume that most people who abandon their kids either do so for a good (or, worse, a bad) reason, or if not, then they are probably suffering from various issues that would make child-rearing a bad idea for them. Would you really want to get to know someone was either mentally disturbed enough, drunk or high enough, or just plain thoughtless enough, to leave a child on a hospital doorstep? What if you were abandoned because they wanted a boy not a girl, because you were not the right colour or race, or because they just thought you were ugly or deformed? I don't even want to think about areas like early sexual abuse, etc.
Yes, there are other, marginally more acceptable, reasons for abandonment (teen pregnancies, rapes, extreme poverty, etc), and yes it may be possible, even laudable, to reach out to such people. But at the back of my mind always is the thought that, if they have made no effort to find you, then they are probably just not interested, and any attempts at reconciliation are probably doomed to embarrassing failure.
At the vey best, it seems to me, any biological parent who abandoned their baby is likely to be somewhat ashamed, even traumatized, by their actions - not a good basis for a relationship. At worst, they will assume you are looking for money, or they will open a channel of communication and then abandon you all over again, or you will cause a seismic shift in an existing happy family (either your own adopted family, or that of the biological parents, or both). I would have thought the odds against a happy outcome are high, that one would be on a proverbial hiding to nothing, and likely implicit in opening a proverbial can of worms.
But how close are my assumptions to the reality?
I was surprised at how difficult it was to find good information on this issue; I had expected to find too much information rather than too little. The issue has been well and truly muddied in recent years with the advent of Facebook and other social media, which allow parents and children searching for their biological families to by-pass the official system to some extent, and on which there are few or no statistics.
This document from the Council of Irish Adoption Agencies, as well as providing some idea of the huge variety in possible outcomes of a search for biological parents, concludes with the rather woolly and inexact: "In general research studies in this area have found that the majority of people who search and have a reunion with a birth relative describe their experience as positive". A 2004 University of British Columbia study suggests that: "Over 90% of all searchers and search subjects reported that reunion was a positive experience." So, maybe I am wrong.
An article in the ever-reliable Guardian seems to me to have hit the nail on the head. Although it reports that about 80% of adoptees and birth parents are happy to have made contact, and that nearly 80% are still in touch eight years later (with only around 7% experiencing outright rejection), the anecdotal evidence seems to belie those statistics to some extent. Of the three cases the article highlights, for example: one adopted child terminated the contact when his birth mother became too needy; one reconnection soured when a biological parent failed to show up for a meeting; and one biological parent ceased contact with her child, "after years of feeling she was taking me out of the cupboard and putting me back again at her will". Yet, significantly, all of these people report their reunions as having been successful, and say that they are glad to have had the experience. And, remember, these are normal adoption-type circumstances, not abandonments.
So, I guess it kind of depends on what you want out of a reunion, and what you consider to be a positive experience.