If you're going to talk about the Book of Revelation, you pretty much have to start by saying that it's a hard read. The NAB's introduction to the Book, for example, begins this way:
The Apocalypse, or Revelation to John, the last book of the Bible, is one of the most difficult to understand because it abounds in unfamiliar and extravagant symbolism, which at best appears unusual to the modern reader.
There are different ways a book can be "a hard read" or "difficult to understand." It can use big words, convoluted syntax, or lots of footnotes. It can be poorly structured, or just organized in an unusual way. It can be about ideas too abstruse, too obscure, or too odd to be grasped by a particular reader.
In that light, Revelation is difficult to understand in an easy sense. It doesn't use big words, convoluted syntax, or footnotes. Its structure is easily discernible -- most likely, it's right there in black and white in your Bible, indicated by headings and possibly an introductory outline. The ideas are familiar to any Catholic who's been paying attention at Mass.
The difficulty of understanding Revelation -- relative to the rest of the Bible, that is; this is, after all, God's self-revelation, not instructions for using a toaster oven -- lies chiefly in the "unfamiliar and extravagant symbolism" mentioned by the NAB.
Alongside the symbolism, though -- and, whatever else you make of it, you pretty much have to know there's something up with the Lamb with seven horns and seven eyes -- the book is filled with allusions to other books of the Bible. Revelation is an eschatological interpretation of human history, so it shouldn't be surprising that it refers back to other interpretations of history, including the prophetic (e.g., Ezekiel) and the, um, historic (e.g., Numbers). But you won't necessarily notice the allusions unless you're already familiar with what they allude to -- or, of course, if they're pointed out to you as you read along.
I'd say the Book of Revelation is "difficult" much the way Cockney rhyming slang is difficult. If I say, "I took the fork to the bath to get a couple of pigs," you won't (you really can't) understand what I mean unless you happen to know that "fork (and knife)" means "wife," "bath (tub)" means "pub," and "pig's (ear)" means "beer." But if you do know these things, my meaning is perfectly clear.
Not that merely knowing the symbolism of Revelation makes the book as transparent as an ordinary human conversation about ordinary human things. But it does make it accessible as Scripture to the ordinary human believer.
To say the Book of Revelation is "difficult to understand," then, is to say that you have to do some work to understand it. At the very least, you have to read a version with good notes and cross-references, and read the notes to understand the symbolism and follow the cross-references to get the allusions. In other words, take advantage of the work other people have already done.