Many many years ago, I read the criticism of one traditionalist Catholic against Medjugorje, and ironically, his nit picking was so overboard that I realized one could say the same for many visionaries and saints. So when I had an opportunity to go there, I did, and was impressed.
This was many years ago, and Yugoslavia was still communist at the time, before the civil war, and the town was starting to be a tourist area, but was still "primitive" in many areas (e.g. pit toilets near the shrine for tourists).
There I found grace.
Many of the criticisms since then (they didn't become nuns or priests) are absurd (neither did the children of LaSalette ) are absurd, given the poor theological state of a lot of religious orders, and the criticism that Mary said "all religions are the same" was a misquote of her saying something to the effect that all religious people were her children, and they shouldn't hate each other. Given the civil war in Bosnia, where secular atheistic Bosnian Orthodox killed sexular atheistic Bosnian Catholics and Secular Atheistic Bosnian Muslims that was indeed an important warning.
But I also remember some other prophecies. Mary prophecized that there would be no nuclear war at a time when this was very possible. She also prophecized that the political problems in Poland would come out well (and in a book by a BBC writer who went there to make a report, the writer sarcastically said: well, what happened in Poland wasn't quite in line with that prophecy, and one year after the book was published, Solidarity won and the iron curtain fell).
Finally, I remember the prophecy that the west was losing it's way, but that Russia would be the one to proclaim Christianity to the world (again I paraphrase).
Still waiting for that one, but with the increasing demonization of Christianity in Europe and the US, it very well may end up being true.
Father Groeshel has an excellent book on apparitions. Some are very obviously Heaven intervening for a big reason, but many are more subtle, where religious folks "see" a vision or "hear" a voice and interpret this from heaven.
The problem? the voices/visions can be medical hallucinations, imagination, mental illness, or real but personal (i.e. for the person involved only, but with their pride, they decide to tell the world and increase their egotism).
In the play ST Joan, GBShaw (an atheist) has her say that everyone hears these voices, but we don't listen.
True. But maybe the voices we hear are not of God.
in Medjugorje, the visionaries remain committed to the visions, even though they are separated and have differing lives. That says a lot.
None of them seem to have gone bonkers or alcoholic, although if they did, it doesn't necessarily mean they were false visionaries (again, the visionaries of La Salette were "failures" as adults).
But the main argument on Medjugorje is that a lot of folks who went there found grace, or even conversion.