Simcha – I don’t know the answer to your question. What I do know is that when I returned to EY recently after being away for several years, I had to figure out which hashgachos I could eat. And since I’m living with family, that was a particularly complicated question.
I wasn’t even sure at first what Rav to ask since there are a lot of different opinions, and I had to ask a Rav who made sense for me and where I am holding Frumkeit-wise and hashkafa-wise, and who would understand my situation. First, I asked several friends what they do. These are all friends whom I trust and whose houses I eat in and who are more stringent than I am when it comes to these types of matters. I then got numbers from them of Rabbanim to ask.
What I found very interesting was exactly the point you are making. There were certain hashagachos that everyone seemed to have similar opinions on (whether that was definitely yes, definitely no, or it’s not the best but you can rely on it if you need to), but there were other hashgachos that there seemed to be very different opinions on.
I’m not sure, but I think that part of this may have had to do with different levels of knowledge regarding the hashgachos. Part of that may have had to do with the fact that these things are always changing, and part of it may have had to do with the fact that there are probably very few people who have really researched the hashgachos and know what is going on. I trusted the Rav I spoke to because I could tell from the way the conversation went that he is a very wise person and he seemed to really know what he is talking about.