I think the "hatred" towards ROAC has little to do with any alleged "canonical" abnormalities (which I can assure you are non-existant - the key is whether or not the "churches" people here are members of, are in communion with heresy or not - if they are not, then ROAC has no right to exist...if they are, it cannot but exist).
If I may speculate...
- for many, it has to do with being accused either of heresy, or at least being in communion with heresy. Generally, people are defensive... it takes a concerted effort not to be, as the "default" for mankind is to be ruled by his passions. I am no better in this regard than anyone here. It's a real struggle. Given this, no one likes to be told they're somehow "in the wrong", let alone in a question like this.
- for others, it is something like this...but their is a more doctrinal stand. For many laymen involved in groups like the GOA, or the Antiochian Archdiocese, etc., there is a simplicity, and probable uneasiness with ecumenism. They'd be perfectly happy to see it come to an end, but feel powerless to do anything about it, or at least their place in it all. However, there are also those (and there are several people like this on this forum) who are die hard ecumenists, and hold to ideas which are, frankly, heretical. At least so says the ROAC (and imho, any number of Church Fathers, Canons, Ecumenical and Pan-Orthodox Synods, etc.), and those holding similar views (at one time, ROCOR, or GOC of Greece, for example). Ecumenism, for whatever reason (sometimes it's due to sentiment, the harsh reality of having family and other loved ones wrapped up in false religion, etc.) is near and dear to their heart. It's the golden calf, and they do not want to part with it. To have someone come out very forcefully, and call such a thing a blasphemy, strikes right at the heart.
This is also all co-mingeled with a great deal of frustration, because it doesn't take very much investigation to realize that the "fanatics" are in fact correct, and becomingly only more manifestly so as the ecumenistic agenda keeps unfolding with each passing day. Frustrating, because there is so little that an ecumenist can authoratatively point to, that defends their innovative positions (grace of heretical mysteries being a big one) - they know the best they can do is point to 20th century "scholarship", and some abbherations of a heavily westernized Russian Orthodox Church of the 19th century (which is odd, since their neo-patristic scholars tend to, otherwise, be the biggest critics of this very obvious westernization...obviously it is a selective criticism), which I might add were various, and troubling, beyond what even most ecumensitic types here would condone (strange neo-gnostic cults were popping up in Russia at this time in an Orthodox guise, and in fact much of this spiritism and strangeness left in parts of the diaspora, and centered in what became the "Paris school"...keep in mind, it was the Russia of this period which gave birth to the "Sophian heresy", touted by proto-ecumenists like Soloviev).
OTOH, the Canons of the Holy Apostles, Canons of St.Basil, various Church Fathers (most famously/notoriously St.Cyprian of Carthage, but also many others - when you read the data on this subject, it becomes very clear how bizarre Pope Stephen's position actually was at the time), the teaching of latter great teachers like St.Nicodemus of the Holy Mountain, etc., all weigh in against these newer opinions. Basically, it's a very uneven "polemic" between these two sides, and the ecumenists generally know this....which is why the more professional ones, play the obscurantist game of not finding "more authoratative" sources than these, but simply burying them in various ways, or more typically, becoming extremely dismissive towards them.
Perhaps the best way to put it for someone coming from your background, is that they act like the "Novus Ordo" folks do - they have a cafeteria approach to the Fathers, and to Christian antiquity in general... hence on one hand, they justify all sorts of liturgical experimentation (talking about the "Orthodox ecumenists" now) on one hand, on the other, they'll basically ignore anything the Fathers say which is not to their liking, or somehow try to undermine it with their allegedly "superior" insights. To give you a good example of what I mean in an area other than ecclessiology (in this case, liturgics), many of the modernistic scholars will encourage the virtual demolition (in some cases, even outright removal) of the iconostasis as it's come down in Orthodox practice, often citing ancient examples when the iconostasis was much simpler, or in some areas, non-existant. Of course, such "liturgists" are extremely selective, since you don't find them suggestion catechumens be removed from the Church before the Holy Oblation, or the complete barring of non-Orthodox from Divine Services...
A similar game is played by these folks with theological matters, though it is generally even more shameless and selective.
One other thing I'd like you to notice Ben, is the difference in the "polemic" of the two sides in this issue. Generally, you'll notice that the "fanatics" (in this case, ROAC, though you'll see the same thing with the Greek Old Calendarists) will deal with personalities (denouncing certain key figures, like Metropolitan Sergius, or one time Ecumenical Patriarch Athenagoras), but there is almost always a doctrinal angle to these denuncations - IOW, the attacks, even involving persons, are rarely limited to their alleged personal failings. Also, you'll notice at the forefront of their argumentation, even before such persons are mentioned, is the doctrinal positions, and their contraries. Their stand is primarily a doctrinal one.
But just look at this thread alone, Ben, let alone others like it on this forum, or on other forums/lists/publications, when it comes to the quality of the argument ecumenists (or their co-religionists who tolerate their activities and teachings) indulge in. How many doctrinal treatments have you seen here, or elsewhere, by these people as to why the doctrine of the ROAC, GOC, etc., are incorrect? The answer is quite obvious, and I would also say, very telling.
Instead, what you will see, are incredible generalizations about people associated with the ROAC ("they're pharisees", "they lack love", etc., etc.), and at best, a lot of hearsay and outright fabrication/distortion regarding the backgrounds of ROAC heirarchs and clergy (and a lot of convienient confusing of facts, for example regarding the person of Metropolitan Valentin of Suzdal, and perhaps the even more disliked Bishop Gregory in the United States.) And, God forbid there actually be (gasp) sinners in the ROAC, or anyone who has ever been guilty of a personal failing at any time - the ad hominem value of things like this is endless (which is odd, since it's supposed to be the ROAC that is loaded with "unforgiving pharisees" and utopian perfectionists...I would figure based on the boasts of ROAC's enemies, they would be of a higher moral and spiritual quality than this, to think a sinner "undoes" an entire Church...which makes me wonder who the real "Donatists" in fact are; a "Donatism of convienience" perhaps?)
As an aside/additional observation... note something very interesting (and it's true, oddly enough, of liberals/people who act in an unprincipeled way, in general).... how you can teach any amount of manifest heresy, be a member of some of the most absurd schisms... yet so long as you are against "intolerance" (translation: a consistant, earnest stance for something as truth, to the exclusion of it's contraries), you are "ok". This is why on this forum, you'll see a cyber-orgy of disparant religious views, and church-affiliations, all patting each other on the back, in their attempt to disparage the ROAC, and the supposed "fanaticism" it represents. Curious, and again, very telling.